Network Working Group                                           E. Rosen
Request for Comments: 4364                           Cisco Systems, Inc.
Obsoletes: 2547                                               Y. Rekhter
Category: Standards Track                         Juniper Networks, Inc.
                                                           February 2006

BGP/MPLS IP Virtual Private Networks (VPNs)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

   This document describes a method by which a Service Provider may use
   an IP backbone to provide IP Virtual Private Networks (VPNs) for its
   customers.  This method uses a "peer model", in which the customers'
   edge routers (CE routers) send their routes to the Service Provider's
   edge routers (PE routers); there is no "overlay" visible to the
   customer's routing algorithm, and CE routers at different sites do
   not peer with each other.  Data packets are tunneled through the
   backbone, so that the core routers do not need to know the VPN
   routes.

   This document obsoletes RFC 2547.

   1. Introduction ....................................................3
      1.1. Virtual Private Networks ...................................4
      1.2. Customer Edge and Provider Edge ............................5
      1.3. VPNs with Overlapping Address Spaces .......................6
      1.4. VPNs with Different Routes to the Same System ..............7
      1.5. SP Backbone Routers ........................................7
      1.6. Security ...................................................8
   2. Sites and CEs ...................................................8
   3. VRFs: Multiple Forwarding Tables in PEs .........................9
      3.1. VRFs and Attachment Circuits ...............................9
      3.2. Associating IP Packets with VRFs ..........................10
      3.3. Populating the VRFs .......................................11
   4. VPN Route Distribution via BGP .................................12
      4.1. The VPN-IPv4 Address Family ...............................13
      4.2. Encoding of Route Distinguishers ..........................14
      4.3. Controlling Route Distribution ............................15
           4.3.1. The Route Target Attribute .........................15
           4.3.2. Route Distribution Among PEs by BGP ................17
           4.3.3. Use of Route Reflectors ............................20
           4.3.4. How VPN-IPv4 NLRI Is Carried in BGP ................22
           4.3.5. Building VPNs Using Route Targets ..................23
           4.3.6. Route Distribution Among VRFs in a Single PE .......23
   5. Forwarding .....................................................23
   6. Maintaining Proper Isolation of VPNs ...........................26
   7. How PEs Learn Routes from CEs ..................................27
   8. How CEs Learn Routes from PEs ..................................30
   9. Carriers' Carriers .............................................30
   10. Multi-AS Backbones ............................................32
   11. Accessing the Internet from a VPN .............................34
   12. Management VPNs ...............................................36
   13. Security Considerations .......................................37
      13.1. Data Plane ...............................................37
      13.2. Control Plane ............................................39
      13.3. Security of P and PE Devices .............................39
   14. Quality of Service ............................................39
   15. Scalability ...................................................40
   16. IANA Considerations ...........................................40
   17. Acknowledgements ..............................................41
   18. Contributors ..................................................41
   19. Normative References ..........................................44
   20. Informative References ........................................45

1. Introduction

   This document describes a method by which a Service Provider may use
   an IP backbone to provide IP Virtual Private Networks (VPNs) for its
   customers.  This method uses a "peer model", in which the customers'
   edge routers (CE routers) send their routes to the Service Provider's
   edge routers (PE routers).  Border Gateway Protocol (BGP)
   [BGP, BGP-MP] is then used by the Service Provider to exchange the
   routes of a particular VPN among the PE routers that are attached to
   that VPN.  This is done in a way that ensures that routes from
   different VPNs remain distinct and separate, even if two VPNs have an
   overlapping address space.  The PE routers distribute, to the CE
   routers in a particular VPN, the routes from other the CE routers in
   that VPN.  The CE routers do not peer with each other, hence there is
   no "overlay" visible to the VPN's routing algorithm.  The term "IP"
   in "IP VPN" is used to indicate that the PE receives IP datagrams
   from the CE, examines their IP headers, and routes them accordingly.

   Each route within a VPN is assigned a Multiprotocol Label Switching
   (MPLS) [MPLS-ARCH, MPLS-BGP, MPLS-ENCAPS] label; when BGP distributes
   a VPN route, it also distributes an MPLS label for that route.
   Before a customer data packet travels across the Service Provider's
   backbone, it is encapsulated with the MPLS label that corresponds, in
   the customer's VPN, to the route that is the best match to the
   packet's destination address.  This MPLS packet is further
   encapsulated (e.g., with another MPLS label or with an IP or Generic
   Routing Encapsulation (GRE) tunnel header [MPLS-in-IP-GRE]) so that
   it gets tunneled across the backbone to the proper PE router.  Thus,
   the backbone core routers do not need to know the VPN routes.

   The primary goal of this method is to support the case in which a
   client obtains IP backbone services from a Service Provider or
   Service Providers with which it maintains contractual relationships.
   The client may be an enterprise, a group of enterprises that need an
   extranet, an Internet Service Provider, an application service
   provider, another VPN Service Provider that uses this same method to
   offer VPNs to clients of its own, etc.  The method makes it very
   simple for the client to use the backbone services.  It is also very
   scalable and flexible for the Service Provider, and allows the
   Service Provider to add value.

1.1. Virtual Private Networks

   Consider a set of "sites" that are attached to a common network that
   we call "the backbone".  Now apply some policy to create a number of
   subsets of that set, and impose the following rule: two sites may
   have IP interconnectivity over that backbone only if at least one of
   these subsets contains them both.

   These subsets are Virtual Private Networks (VPNs).  Two sites have IP
   connectivity over the common backbone only if there is some VPN that
   contains them both.  Two sites that have no VPN in common have no
   connectivity over that backbone.

   If all the sites in a VPN are owned by the same enterprise, the VPN
   may be thought of as a corporate "intranet".  If the various sites in
   a VPN are owned by different enterprises, the VPN may be thought of
   as an "extranet".  A site can be in more than one VPN; e.g., in an
   intranet and in several extranets.  In general, when we use the term
   "VPN" we will not be distinguishing between intranets and extranets.

   We refer to the owners of the sites as the "customers".  We refer to
   the owners/operators of the backbone as the "Service Providers"
   (SPs).  The customers obtain "VPN service" from the SPs.

   A customer may be a single enterprise, a set of enterprises, an
   Internet Service Provider, an Application Service Provider, another
   SP that offers the same kind of VPN service to its own customers,
   etc.

   The policies that determine whether a particular collection of sites
   is a VPN are the policies of the customers.  Some customers will want
   the implementation of these policies to be entirely the
   responsibility of the SP.  Other customers may want to share with the
   SP the responsibility for implementing these policies.  This document
   specifies mechanisms that can be used to implement these policies.
   The mechanisms we describe are general enough to allow these policies
   to be implemented either by the SP alone or by a VPN customer
   together with the SP.  Most of the discussion is focused on the
   former case, however.

   The mechanisms discussed in this document allow the implementation of
   a wide range of policies.  For example, within a given VPN, one can
   allow every site to have a direct route to every other site ("full
   mesh").  Alternatively, one can force traffic between certain pairs
   of sites to be routed via a third site.  This can be useful, e.g., if
   it is desired that traffic between a pair of sites be passed through
   a firewall, and the firewall is located at the third site.

   In this document, we restrict our discussion to the case in which the
   customer is explicitly purchasing VPN service from an SP, or from a
   set of SPs that have agreed to cooperate to provide the VPN service.
   That is, the customer is not merely purchasing internet access from
   an SP, and the VPN traffic does not pass through a random collection
   of interconnected SP networks.

   We also restrict our discussion to the case in which the backbone
   provides an IP service to the customer, rather than, e.g., a layer 2
   service such as Frame Relay, Asynchronous Transfer Mode (ATM),
   ethernet, High Level Data Link Control (HDLC), or Point-to-Point
   Protocol (PPP).  The customer may attach to the backbone via one of
   these (or other) layer 2 services, but the layer 2 service is
   terminated at the "edge" of the backbone, where the customer's IP
   datagrams are removed from any layer 2 encapsulation.

   In the rest of this introduction, we specify some properties that
   VPNs should have.  The remainder of this document specifies a set of
   mechanisms that can be deployed to provide a VPN model that has all
   these properties.  This section also introduces some of the technical
   terminology used in the remainder of the document.

1.2. Customer Edge and Provider Edge

   Routers can be attached to each other, or to end systems, in a
   variety of different ways: PPP connections, ATM Virtual Circuits
   (VCs), Frame Relay VCs, ethernet interfaces, Virtual Local Area
   Networks (VLANs) on ethernet interfaces, GRE tunnels, Layer 2
   Tunneling Protocol (L2TP) tunnels, IPsec tunnels, etc.  We will use
   the term "attachment circuit" to refer generally to some such means
   of attaching to a router.  An attachment circuit may be the sort of
   connection that is usually thought of as a "data link", or it may be
   a tunnel of some sort; what matters is that it be possible for two
   devices to be network layer peers over the attachment circuit.

   Each VPN site must contain one or more Customer Edge (CE) devices.
   Each CE device is attached, via some sort of attachment circuit, to
   one or more Provider Edge (PE) routers.

   Routers in the SP's network that do not attach to CE devices are
   known as "P routers".

   CE devices can be hosts or routers.  In a typical case, a site
   contains one or more routers, some of which are attached to PE
   routers.  The site routers that attach to the PE routers would then
   be the CE devices, or "CE routers".  However, there is nothing to
   prevent a non-routing host from attaching directly to a PE router, in
   which case the host would be a CE device.

   Sometimes, what is physically attached to a PE router is a layer 2
   switch.  In this case, we do NOT say that the layer 2 switch is a CE
   device.  Rather, the CE devices are the hosts and routers that
   communicate with the PE router through the layer 2 switch; the layer
   2 infrastructure is transparent.  If the layer 2 infrastructure
   provides a multipoint service, then multiple CE devices can be
   attached to the PE router over the same attachment circuit.

   CE devices are logically part of a customer's VPN.  PE and P routers
   are logically part of the SP's network.

   The attachment circuit over which a packet travels when going from CE
   to PE is known as that packet's "ingress attachment circuit", and the
   PE as the packet's "ingress PE".  The attachment circuit over which a
   packet travels when going from PE to CE is known as that packet's
   "egress attachment circuit", and the PE as the packet's "egress PE".

   We will say that a PE router is attached to a particular VPN if it is
   attached to a CE device that is in a site of that VPN.  Similarly, we
   will say that a PE router is attached to a particular site if it is
   attached to a CE device that is in that site.

   When the CE device is a router, it is a routing peer of the PE(s) to
   which it is attached, but it is NOT a routing peer of CE routers at
   other sites.  Routers at different sites do not directly exchange
   routing information with each other; in fact, they do not even need
   to know of each other at all.  As a consequence, the customer has no
   backbone or "virtual backbone" to manage, and does not have to deal
   with any inter-site routing issues.  In other words, in the scheme
   described in this document, a VPN is NOT an "overlay" on top of the
   SP's network.

   With respect to the management of the edge devices, clear
   administrative boundaries are maintained between the SP and its
   customers.  Customers are not required to access the PE or P routers
   for management purposes, nor is the SP required to access the CE
   devices for management purposes.

1.3. VPNs with Overlapping Address Spaces

   If two VPNs have no sites in common, then they may have overlapping
   address spaces.  That is, a given address might be used in VPN V1 as
   the address of system S1, but in VPN V2 as the address of a
   completely different system S2.  This is a common situation when the
   VPNs each use an RFC 1918 private address space.  Of course, within
   each VPN, each address must be unambiguous.

   Even two VPNs that do have sites in common may have overlapping
   address spaces, as long as there is no need for any communication
   between systems with such addresses and systems in the common sites.

1.4. VPNs with Different Routes to the Same System

   Although a site may be in multiple VPNs, it is not necessarily the
   case that the route to a given system at that site should be the same
   in all the VPNs.  Suppose, for example, we have an intranet
   consisting of sites A, B, and C, and an extranet consisting of A, B,
   C, and the "foreign" site D.  Suppose that at site A there is a
   server, and we want clients from B, C, or D to be able to use that
   server.  Suppose also that at site B there is a firewall.  We want
   all the traffic from site D to the server to pass through the
   firewall, so that traffic from the extranet can be access controlled.
   However, we don't want traffic from C to pass through the firewall on
   the way to the server, since this is intranet traffic.

   It is possible to set up two routes to the server.  One route, used
   by sites B and C, takes the traffic directly to site A.  The second
   route, used by site D, takes the traffic instead to the firewall at
   site B.  If the firewall allows the traffic to pass, it then appears
   to be traffic coming from site B, and follows the route to site A.

1.5. SP Backbone Routers

   The SP's backbone consists of the PE routers, as well as other
   routers ("P routers") that do not attach to CE devices.

   If every router in an SP's backbone had to maintain routing
   information for all the VPNs supported by the SP, there would be
   severe scalability problems; the number of sites that could be
   supported would be limited by the amount of routing information that
   could be held in a single router.  It is important therefore that the
   routing information about a particular VPN only needs to be present
   in the PE routers that attach to that VPN.  In particular, the P
   routers do not need to have ANY per-VPN routing information
   whatsoever.  (This condition may need to be relaxed somewhat when
   multicast routing is considered.  This is not considered further in
   this paper, but is examined in [VPN-MCAST].)

   So just as the VPN owners do not have a backbone or "virtual
   backbone" to administer, the SPs themselves do not have a separate
   backbone or "virtual backbone" to administer for each VPN.  Site-to-
   site routing in the backbone is optimal (within the constraints of
   the policies used to form the VPNs) and is not constrained in any way
   by an artificial "virtual topology" of tunnels.

Section 10 discusses some of the special issues that arise when the
backbone spans several Service Providers.

1.6. Security

   VPNs of the sort being discussed here, even without making use of
   cryptographic security measures, are intended to provide a level of
   security equivalent to that obtainable when a layer 2 backbone (e.g.,
   Frame Relay) is used.  That is, in the absence of misconfiguration or
   deliberate interconnection of different VPNs, it is not possible for
   systems in one VPN to gain access to systems in another VPN.  Of
   course, the methods described herein do not by themselves encrypt the
   data for privacy, nor do they provide a way to determine whether data
   has been tampered with en route.  If this is desired, cryptographic
   measures must be applied in addition. (See, e.g., [MPLS/BGP-IPsec].)
   Security is discussed in more detail in Section 13.

2. Sites and CEs

   From the perspective of a particular backbone network, a set of IP
   systems may be regarded as a "site" if those systems have mutual IP
   interconnectivity that doesn't require use of the backbone.  In
   general, a site will consist of a set of systems that are in
   geographic proximity.  However, this is not universally true.  If two
   geographic locations are connected via a leased line, over which Open
   Shortest Path First (OSPF) protocol [OSPFv2] is running, and if that
   line is the preferred way of communicating between the two locations,
   then the two locations can be regarded as a single site, even if each
   location has its own CE router.  (This notion of "site" is
   topological, rather than geographical.  If the leased line goes down,
   or otherwise ceases to be the preferred route, but the two geographic
   locations can continue to communicate by using the VPN backbone, then
   one site has become two.)

   A CE device is always regarded as being in a single site (though as
   we shall see in Section 3.2, a site may consist of multiple "virtual
   sites").  A site, however, may belong to multiple VPNs.

   A PE router may attach to CE devices from any number of different
   sites, whether those CE devices are in the same or in different VPNs.
   A CE device may, for robustness, attach to multiple PE routers, of
   the same or of different service providers.  If the CE device is a
   router, the PE router and the CE router will appear as router
   adjacencies to each other.

   While we speak mostly of "sites" as being the basic unit of
   interconnection, nothing here prevents a finer degree of granularity
   in the control of interconnectivity.  For example, certain systems at

   a site may be members of an intranet as well as members of one or
   more extranets, while other systems at the same site may be
   restricted to being members of the intranet only.  However, this
   might require that the site have two attachment circuits to the
   backbone, one for the intranet and one for the extranet; it might
   further require that firewall functionality be applied on the
   extranet attachment circuit.

3. VRFs: Multiple Forwarding Tables in PEs

   Each PE router maintains a number of separate forwarding tables.  One
   of the forwarding tables is the "default forwarding table".  The
   others are "VPN Routing and Forwarding tables", or "VRFs".

3.1. VRFs and Attachment Circuits

   Every PE/CE attachment circuit is associated, by configuration, with
   one or more VRFs.  An attachment circuit that is associated with a
   VRF is known as a "VRF attachment circuit".

   In the simplest case and most typical case, a PE/CE attachment
   circuit is associated with exactly one VRF.  When an IP packet is
   received over a particular attachment circuit, its destination IP
   address is looked up in the associated VRF.  The result of that
   lookup determines how to route the packet.  The VRF used by a
   packet's ingress PE for routing a particular packet is known as the
   packet's "ingress VRF".  (There is also the notion of a packet's
   "egress VRF", located at the packet's egress PE; this is discussed in
   Section 5.)

   If an IP packet arrives over an attachment circuit that is not
   associated with any VRF, the packet's destination address is looked
   up in the default forwarding table, and the packet is routed
   accordingly.  Packets forwarded according to the default forwarding
   table include packets from neighboring P or PE routers, as well as
   packets from customer-facing attachment circuits that have not been
   associated with VRFs.

   Intuitively, one can think of the default forwarding table as
   containing "public routes", and of the VRFs as containing "private
   routes".  One can similarly think of VRF attachment circuits as being
   "private", and of non-VRF attachment circuits as being "public".

   If a particular VRF attachment circuit connects site S to a PE
   router, then connectivity from S (via that attachment circuit) can be
   restricted by controlling the set of routes that gets entered in the
   corresponding VRF.  The set of routes in that VRF should be limited
   to the set of routes leading to sites that have at least one VPN in

   common with S.  Then a packet sent from S over a VRF attachment
   circuit can only be routed by the PE to another site S' if S' is in
   one of the same VPNs as S.  That is, communication (via PE routers)
   is prevented between any pair of VPN sites that have no VPN in
   common.  Communication between VPN sites and non-VPN sites is
   prevented by keeping the routes to the VPN sites out of the default
   forwarding table.

   If there are multiple attachment circuits leading from S to one or
   more PE routers, then there might be multiple VRFs that could be used
   to route traffic from S.  To properly restrict S's connectivity, the
   same set of routes would have to exist in all the VRFs.
   Alternatively, one could impose different connectivity restrictions
   over different attachment circuit from S.  In that case, some of the
   VRFs associated with attachment circuits from S would contain
   different sets of routes than some of the others.

   We allow the case in which a single attachment circuit is associated
   with a set of VRFs, rather than with a single VRF.  This can be
   useful if it is desired to divide a single VPN into several
   "sub-VPNs", each with different connectivity restrictions, where some
   characteristic of the customer packets is used to select from among
   the sub-VPNs.  For simplicity though, we will usually speak of an
   attachment circuit as being associated with a single VRF.

3.2. Associating IP Packets with VRFs

   When a PE router receives a packet from a CE device, it must
   determine the attachment circuit over which the packet arrived, as
   this determines in turn the VRF (or set of VRFs) that can be used for
   forwarding that packet.  In general, to determine the attachment
   circuit over which a packet arrived, a PE router takes note of the
   physical interface over which the packet arrived, and possibly also
   takes note of some aspect of the packet's layer 2 header.  For
   example, if a packet's ingress attachment circuit is a Frame Relay
   VC, the identity of the attachment circuit can be determined from the
   physical Frame Relay interface over which the packet arrived,
   together with the Data Link Connection Identifier (DLCI) field in the
   packet's Frame Relay header.

   Although the PE's conclusion that a particular packet arrived on a
   particular attachment circuit may be partially determined by the
   packet's layer 2 header, it must be impossible for a customer, by
   writing the header fields, to fool the SP into thinking that a packet
   that was received over one attachment circuit really arrived over a
   different one.  In the example above, although the attachment circuit
   is determined partially by inspection of the DLCI field in the Frame
   Relay header, this field cannot be set freely by the customer.

   Rather, it must be set to a value specified by the SP, or else the
   packet cannot arrive at the PE router.

   In some cases, a particular site may be divided by the customer into
   several "virtual sites".  The SP may designate a particular set of
   VRFs to be used for routing packets from that site and may allow the
   customer to set some characteristic of the packet, which is then used
   for choosing a particular VRF from the set.

   For example, each virtual site might be realized as a VLAN.  The SP
   and the customer could agree that on packets arriving from a
   particular CE, certain VLAN values would be used to identify certain
   VRFs.  Of course, packets from that CE would be discarded by the PE
   if they carry VLAN tag values that are not in the agreed-upon set.
   Another way to accomplish this is to use IP source addresses.  In
   this case, the PE uses the IP source address in a packet received
   from the CE, along with the interface over which the packet is
   received, to assign the packet to a particular VRF.  Again, the
   customer would only be able to select from among the particular set
   of VRFs that that customer is allowed to use.

   If it is desired to have a particular host be in multiple virtual
   sites, then that host must determine, for each packet, which virtual
   site the packet is associated with.  It can do this, e.g., by sending
   packets from different virtual sites on different VLANs, or out
   different network interfaces.

3.3. Populating the VRFs

   With what set of routes are the VRFs populated?

   As an example, let PE1, PE2, and PE3 be three PE routers, and let
   CE1, CE2, and CE3 be three CE routers.  Suppose that PE1 learns, from
   CE1, the routes that are reachable at CE1's site.  If PE2 and PE3 are
   attached, respectively, to CE2 and CE3, and there is some VPN V
   containing CE1, CE2, and CE3, then PE1 uses BGP to distribute to PE2
   and PE3 the routes that it has learned from CE1.  PE2 and PE3 use
   these routes to populate the VRFs that they associate, respectively,
   with the sites of CE2 and CE3.  Routes from sites that are not in VPN
   V do not appear in these VRFs, which means that packets from CE2 or
   CE3 cannot be sent to sites that are not in VPN V.

   When we speak of a PE "learning" routes from a CE, we are not
   presupposing any particular learning technique.  The PE may learn
   routes by means of a dynamic routing algorithm, but it may also
   "learn" routes by having those routes configured (i.e., static
   routing).  (In this case, to say that the PE "learned" the routes
   from the CE is perhaps to exercise a bit of poetic license.)

   PEs also need to learn, from other PEs, the routes that belong to a
   given VPN.  The procedures to be used for populating the VRFs with
   the proper sets of routes are specified in Section 4.

   If there are multiple attachment circuits leading from a particular
   PE router to a particular site, they might all be mapped to the same
   forwarding table.  But if policy dictates, they could be mapped to
   different forwarding tables.  For instance, the policy might be that
   a particular attachment circuit from a site is used only for intranet
   traffic, while another attachment circuit from that site is used only
   for extranet traffic.  (Perhaps, e.g., the CE attached to the
   extranet attachment circuit is a firewall, while the CE attached to
   the intranet attachment circuit is not.)  In this case, the two
   attachment circuits would be associated with different VRFs.

   Note that if two attachment circuits are associated with the same
   VRF, then packets that the PE receives over one of them will be able
   to reach exactly the same set of destinations as packets that the PE
   receives over the other.  So two attachment circuits cannot be
   associated with the same VRF unless each CE is in the exact same set
   of VPNs as is the other.

   If an attachment circuit leads to a site which is in multiple VPNs,
   the attachment circuit may still associated with a single VRF, in
   which case the VRF will contain routes from the full set of VPNs of
   which the site is a member.

4. VPN Route Distribution via BGP

   PE routers use BGP to distribute VPN routes to each other (more
   accurately, to cause VPN routes to be distributed to each other).

   We allow each VPN to have its own address space, which means that a
   given address may denote different systems in different VPNs.  If two
   routes to the same IP address prefix are actually routes to different
   systems, it is important to ensure that BGP not treat them as
   comparable.  Otherwise, BGP might choose to install only one of them,
   making the other system unreachable.  Further, we must ensure that
   POLICY is used to determine which packets get sent on which routes;
   given that several such routes are installed by BGP, only one such
   must appear in any particular VRF.

   We meet these goals by the use of a new address family, as specified
   below.

4.1. The VPN-IPv4 Address Family

   The BGP Multiprotocol Extensions [BGP-MP] allow BGP to carry routes
   from multiple "address families".  We introduce the notion of the
   "VPN-IPv4 address family".  A VPN-IPv4 address is a 12-byte quantity,
   beginning with an 8-byte Route Distinguisher (RD) and ending with a
   4-byte IPv4 address.  If several VPNs use the same IPv4 address
   prefix, the PEs translate these into unique VPN-IPv4 address
   prefixes.  This ensures that if the same address is used in several
   different VPNs, it is possible for BGP to carry several completely
   different routes to that address, one for each VPN.

   Since VPN-IPv4 addresses and IPv4 addresses are different address
   families, BGP never treats them as comparable addresses.

   An RD is simply a number, and it does not contain any inherent
   information; it does not identify the origin of the route or the set
   of VPNs to which the route is to be distributed.  The purpose of the
   RD is solely to allow one to create distinct routes to a common IPv4
   address prefix.  Other means are used to determine where to
   redistribute the route (see Section 4.3).

   The RD can also be used to create multiple different routes to the
   very same system.  We have already discussed a situation in which the
   route to a particular server should be different for intranet traffic
   than for extranet traffic.  This can be achieved by creating two
   different VPN-IPv4 routes that have the same IPv4 part, but different
   RDs.  This allows BGP to install multiple different routes to the
   same system, and allows policy to be used (see Section 4.3.5) to
   decide which packets use which route.

   The RDs are structured so that every Service Provider can administer
   its own "numbering space" (i.e., can make its own assignments of
   RDs), without conflicting with the RD assignments made by any other
   Service Provider.  An RD consists of three fields: a 2-byte type
   field, an administrator field, and an assigned number field.  The
   value of the type field determines the lengths of the other two
   fields, as well as the semantics of the administrator field.  The
   administrator field identifies an assigned number authority, and the
   assigned number field contains a number that has been assigned, by
   the identified authority, for a particular purpose.  For example, one
   could have an RD whose administrator field contains an Autonomous
   System number (ASN), and whose (4-byte) number field contains a
   number assigned by the SP to whom that ASN belongs (having been
   assigned to that SP by the appropriate authority).

   RDs are given this structure in order to ensure that an SP that
   provides VPN backbone service can always create a unique RD when it

   needs to do so.  However, the structure is not meaningful to BGP;
   when BGP compares two such address prefixes, it ignores the structure
   entirely.

   A PE needs to be configured such that routes that lead to a
   particular CE become associated with a particular RD.  The
   configuration may cause all routes leading to the same CE to be
   associated with the same RD, or it may cause different routes to be
   associated with different RDs, even if they lead to the same CE.

4.2. Encoding of Route Distinguishers

   As stated, a VPN-IPv4 address consists of an 8-byte Route
   Distinguisher followed by a 4-byte IPv4 address.  The RDs are encoded
   as follows:

     - Type Field: 2 bytes
     - Value Field: 6 bytes

   The interpretation of the Value field depends on the value of the
   type field.  At the present time, three values of the type field are
   defined: 0, 1, and 2.

     - Type 0: The Value field consists of two subfields:

         * Administrator subfield: 2 bytes
         * Assigned Number subfield: 4 bytes

       The Administrator subfield must contain an Autonomous System
       number.  If this ASN is from the public ASN space, it must have
       been assigned by the appropriate authority (use of ASN values
       from the private ASN space is strongly discouraged).  The
       Assigned Number subfield contains a number from a numbering space
       that is administered by the enterprise to which the ASN has been
       assigned by an appropriate authority.

     - Type 1: The Value field consists of two subfields:

         * Administrator subfield: 4 bytes
         * Assigned Number subfield: 2 bytes

       The Administrator subfield must contain an IP address.  If this
       IP address is from the public IP address space, it must have been
       assigned by an appropriate authority (use of addresses from the
       private IP address space is strongly discouraged).  The Assigned
       Number subfield contains a number from a numbering space which is
       administered by the enterprise to which the IP address has been
       assigned.

     - Type 2: The Value field consists of two subfields:

         * Administrator subfield: 4 bytes
         * Assigned Number subfield: 2 bytes

       The Administrator subfield must contain a 4-byte Autonomous
       System number [BGP-AS4].  If this ASN is from the public ASN
       space, it must have been assigned by the appropriate authority
       (use of ASN values from the private ASN space is strongly
       discouraged).  The Assigned Number subfield contains a number
       from a numbering space which is administered by the enterprise to
       which the ASN has been assigned by an appropriate authority.

4.3. Controlling Route Distribution

   In this section, we discuss the way in which the distribution of the
   VPN-IPv4 routes is controlled.

   If a PE router is attached to a particular VPN (by being attached to
   a particular CE in that VPN), it learns some of that VPN's IP routes
   from the attached CE router.  Routes learned from a CE routing peer
   over a particular attachment circuit may be installed in the VRF
   associated with that attachment circuit.  Exactly which routes are
   installed in this manner is determined by the way in which the PE
   learns routes from the CE.  In particular, when the PE and CE are
   routing protocol peers, this is determined by the decision process of
   the routing protocol; this is discussed in Section 7.

   These routes are then converted to VPN-IP4 routes, and "exported" to
   BGP.  If there is more than one route to a particular VPN-IP4 address
   prefix, BGP chooses the "best" one, using the BGP decision process.
   That route is then distributed by BGP to the set of other PEs that
   need to know about it.  At these other PEs, BGP will again choose the
   best route for a particular VPN-IP4 address prefix.  Then the chosen
   VPN-IP4 routes are converted back into IP routes, and "imported" into
   one or more VRFs.  Whether they are actually installed in the VRFs
   depends on the decision process of the routing method used between
   the PE and those CEs that are associated with the VRF in question.
   Finally, any route installed in a VRF may be distributed to the
   associated CE routers.

4.3.1. The Route Target Attribute

   Every VRF is associated with one or more Route Target (RT)
   attributes.

   When a VPN-IPv4 route is created (from an IPv4 route that the PE has
   learned from a CE) by a PE router, it is associated with one or more

   Route Target attributes.  These are carried in BGP as attributes of
   the route.

   Any route associated with Route Target T must be distributed to every
   PE router that has a VRF associated with Route Target T.  When such a
   route is received by a PE router, it is eligible to be installed in
   those of the PE's VRFs that are associated with Route Target T.
   (Whether it actually gets installed depends upon the outcome of the
   BGP decision process, and upon the outcome of the decision process of
   the IGP (i.e., the intra-domain routing protocol) running on the
   PE/CE interface.)

   A Route Target attribute can be thought of as identifying a set of
   sites.  (Though it would be more precise to think of it as
   identifying a set of VRFs.)  Associating a particular Route Target
   attribute with a route allows that route to be placed in the VRFs
   that are used for routing traffic that is received from the
   corresponding sites.

   There is a set of Route Targets that a PE router attaches to a route
   received from site S; these may be called the "Export Targets".  And
   there is a set of Route Targets that a PE router uses to determine
   whether a route received from another PE router could be placed in
   the VRF associated with site S; these may be called the "Import
   Targets".  The two sets are distinct, and need not be the same.  Note
   that a particular VPN-IPv4 route is only eligible for installation in
   a particular VRF if there is some Route Target that is both one of
   the route's Route Targets and one of the VRF's Import Targets.

   The function performed by the Route Target attribute is similar to
   that performed by the BGP Communities attribute.  However, the format
   of the latter is inadequate for present purposes, since it allows
   only a 2-byte numbering space.  It is desirable to structure the
   format, similar to what we have described for RDs (see Section 4.2),
   so that a type field defines the length of an administrator field,
   and the remainder of the attribute is a number from the specified
   administrator's numbering space.  This can be done using BGP Extended
   Communities.  The Route Targets discussed herein are encoded as BGP
   Extended Community Route Targets [BGP-EXTCOMM].  They are structured
   similarly to the RDs.

   When a BGP speaker has received more than one route to the same VPN-
   IPv4 prefix, the BGP rules for route preference are used to choose
   which VPN-IPv4 route is installed by BGP.

   Note that a route can only have one RD, but it can have multiple
   Route Targets.  In BGP, scalability is improved if one has a single
   route with multiple attributes, as opposed to multiple routes.  One

   could eliminate the Route Target attribute by creating more routes
   (i.e., using more RDs), but the scaling properties would be less
   favorable.

   How does a PE determine which Route Target attributes to associate
   with a given route?  There are a number of different possible ways.
   The PE might be configured to associate all routes that lead to a
   specified site with a specified Route Target.  Or the PE might be
   configured to associate certain routes leading to a specified site
   with one Route Target, and certain with another.

   If the PE and the CE are themselves BGP peers (see Section 7), then
   the SP may allow the customer, within limits, to specify how its
   routes are to be distributed.  The SP and the customer would need to
   agree in advance on the set of RTs that are allowed to be attached to
   the customer's VPN routes.  The CE could then attach one or more of
   those RTs to each IP route that it distributes to the PE.  This gives
   the customer the freedom to specify in real time, within agreed-upon
   limits, its route distribution policies.  If the CE is allowed to
   attach RTs to its routes, the PE MUST filter out all routes that
   contain RTs that the customer is not allowed to use.  If the CE is
   not allowed to attach RTs to its routes, but does so anyway, the PE
   MUST remove the RT before converting the customer's route to a VPN-
   IPv4 route.

4.3.2. Route Distribution Among PEs by BGP

   If two sites of a VPN attach to PEs that are in the same Autonomous
   System, the PEs can distribute VPN-IPv4 routes to each other by means
   of an IBGP connection between them.  (The term "IBGP" refers to the
   set of protocols and procedures used when there is a BGP connection
   between two BGP speakers in the same Autonomous System.  This is
   distinguished from "EBGP", the set of procedures used between two BGP
   speakers in different Autonomous Systems.)  Alternatively, each can
   have an IBGP connection to a route reflector [BGP-RR].

   When a PE router distributes a VPN-IPv4 route via BGP, it uses its
   own address as the "BGP next hop".  This address is encoded as a
   VPN-IPv4 address with an RD of 0.  ([BGP-MP] requires that the next
   hop address be in the same address family as the Network Layer
   Reachability Information (NLRI).)  It also assigns and distributes an
   MPLS label.  (Essentially, PE routers distribute not VPN-IPv4 routes,
   but Labeled VPN-IPv4 routes.  Cf. [MPLS-BGP].)  When the PE processes
   a received packet that has this label at the top of the stack, the PE
   will pop the stack, and process the packet appropriately.

   The PE may distribute the exact set of routes that appears in the
   VRF, or it may perform summarization and distribute aggregates of
   those routes, or it may do some of one and some of the other.

   Suppose that a PE has assigned label L to route R, and has
   distributed this label mapping via BGP.  If R is an aggregate of a
   set of routes in the VRF, the PE will know that packets from the
   backbone that arrive with this label must have their destination
   addresses looked up in a VRF.  When the PE looks up the label in its
   Label Information Base, it learns which VRF must be used.  On the
   other hand, if R is not an aggregate, then when the PE looks up the
   label, it learns the egress attachment circuit, as well as the
   encapsulation header for the packet.  In this case, no lookup in the
   VRF is done.

   We would expect that the most common case would be the case where the
   route is NOT an aggregate.  The case where it is an aggregate can be
   very useful though if the VRF contains a large number of host routes
   (e.g., as in dial-in), or if the VRF has an associated Local Area
   Network (LAN) interface (where there is a different outgoing layer 2
   header for each system on the LAN, but a route is not distributed for
   each such system).

   Whether or not each route has a distinct label is an implementation
   matter.  There are a number of possible algorithms one could use to
   determine whether two routes get assigned the same label:

     - One may choose to have a single label for an entire VRF, so that
       a single label is shared by all the routes from that VRF.  Then
       when the egress PE receives a packet with that label, it must
       look up the packet's IP destination address in that VRF (the
       packet's "egress VRF"), in order to determine the packet's egress
       attachment circuit and the corresponding data link encapsulation.

     - One may choose to have a single label for each attachment
       circuit, so that a single label is shared by all the routes with
       the same "outgoing attachment circuit".  This enables one to
       avoid doing a lookup in the egress VRF, though some sort of
       lookup may need to be done in order to determine the data link
       encapsulation, e.g., an Address Resolution Protocol (ARP) lookup.

     - One may choose to have a distinct label for each route.  Then if
       a route is potentially reachable over more than one attachment
       circuit, the PE/CE routing can switch the preferred path for a
       route from one attachment circuit to another, without there being
       any need to distribute new a label for that route.

   There may be other possible algorithms as well.  The choice of
   algorithm is entirely at the discretion of the egress PE, and is
   otherwise transparent.

   In using BGP-distributed MPLS labels in this manner, we presuppose
   that an MPLS packet carrying such a label can be tunneled from the
   router that installs the corresponding BGP-distributed route to the
   router that is the BGP next hop of that route.  This requires either
   that a label switched path exist between those two routers or else
   that some other tunneling technology (e.g., [MPLS-in-IP-GRE]) can be
   used between them.

   This tunnel may follow a "best effort" route, or it may follow a
   traffic-engineered route.  Between a given pair of routers, there may
   be one such tunnel, or there may be several, perhaps with different
   Quality of Service (QoS) characteristics.  All that matters for the
   VPN architecture is that some such tunnel exists.  To ensure
   interoperability among systems that implement this VPN architecture
   using MPLS label switched paths as the tunneling technology, all such
   systems MUST support Label Distribution Protocol (LDP) [MPLS-LDP].
   In particular, Downstream Unsolicited mode MUST be supported on
   interfaces that are neither Label Controlled ATM (LC-ATM) [MPLS-ATM]
   nor Label Controlled Frame Relay (LC-FR) [MPLS-FR] interfaces, and
   Downstream on Demand mode MUST be supported on LC-ATM interfaces and
   LC-FR interfaces.

   If the tunnel follows a best-effort route, then the PE finds the
   route to the remote endpoint by looking up its IP address in the
   default forwarding table.

   A PE router, UNLESS it is a route reflector (see Section 4.3.3) or an
   Autonomous System Border Router (ASBR) for an inter-provider VPN (see
   Section 10), should not install a VPN-IPv4 route unless it has at
   least one VRF with an Import Target identical to one of the route's
   Route Target attributes.  Inbound filtering should be used to cause
   such routes to be discarded.  If a new Import Target is later added
   to one of the PE's VRFs (a "VPN Join" operation), it must then
   acquire the routes it may previously have discarded.  This can be
   done using the refresh mechanism described in [BGP-RFSH].  The
   outbound route filtering mechanism of [BGP-ORF] can also be used to
   advantage to make the filtering more dynamic.

   Similarly, if a particular Import Target is no longer present in any
   of a PE's VRFs (as a result of one or more "VPN Prune" operations),
   the PE may discard all routes that, as a result, no longer have any
   of the PE's VRF's Import Targets as one of their Route Target
   attributes.

   A router that is not attached to any VPN and that is not a Route
   Reflector (i.e., a P router) never installs any VPN-IPv4 routes at
   all.

   Note that VPN Join and Prune operations are non-disruptive and do not
   require any BGP connections to be brought down, as long as the
   refresh mechanism of [BGP-RFSH] is used.

   As a result of these distribution rules, no one PE ever needs to
   maintain all routes for all VPNs; this is an important scalability
   consideration.

4.3.3. Use of Route Reflectors

   Rather than having a complete IBGP mesh among the PEs, it is
   advantageous to make use of BGP Route Reflectors [BGP-RR] to improve
   scalability.  All the usual techniques for using route reflectors to
   improve scalability (e.g., route reflector hierarchies) are
   available.

   Route reflectors are the only systems that need to have routing
   information for VPNs to which they are not directly attached.
   However, there is no need to have any one route reflector know all
   the VPN-IPv4 routes for all the VPNs supported by the backbone.

   We outline below two different ways to partition the set of VPN-IPv4
   routes among a set of route reflectors.

      1. Each route reflector is preconfigured with a list of Route
         Targets.  For redundancy, more than one route reflector may be
         preconfigured with the same list.  A route reflector uses the
         preconfigured list of Route Targets to construct its inbound
         route filtering.  The route reflector may use the techniques of
         [BGP-ORF] to install on each of its peers (regardless of
         whether the peer is another route reflector or a PE) the set of
         Outbound Route Filters (ORFs) that contains the list of its
         preconfigured Route Targets.  Note that route reflectors should
         accept ORFs from other route reflectors, which means that route
         reflectors should advertise the ORF capability to other route
         reflectors.

         A service provider may modify the list of preconfigured Route
         Targets on a route reflector.  When this is done, the route
         reflector modifies the ORFs it installs on all of its IBGP
         peers.  To reduce the frequency of configuration changes on
         route reflectors, each route reflector may be preconfigured
         with a block of Route Targets.  This way, when a new Route
         Target is needed for a new VPN, there is already one or more

         route reflectors that are (pre)configured with this Route
         Target.

         Unless a given PE is a client of all route reflectors, when a
         new VPN is added to the PE ("VPN Join"), it will need to become
         a client of the route reflector(s) that maintain routes for
         that VPN.  Likewise, deleting an existing VPN from the PE ("VPN
         Prune") may result in a situation where the PE no longer needs
         to be a client of some route reflector(s).  In either case, the
         Join or Prune operation is non-disruptive (as long as
         [BGP-RFSH] is used, and never requires a BGP connection to be
         brought down, only to be brought right back up.

         (By "adding a new VPN to a PE", we really mean adding a new
         import Route Target to one of its VRFs, or adding a new VRF
         with an import Route Target not had by any of the PE's other
         VRFs.)

      2. Another method is to have each PE be a client of some subset of
         the route reflectors.  A route reflector is not preconfigured
         with the list of Route Targets, and does not perform inbound
         route filtering of routes received from its clients (PEs);
         rather, it accepts all the routes received from all of its
         clients (PEs).  The route reflector keeps track of the set of
         the Route Targets carried by all the routes it receives.  When
         the route reflector receives from its client a route with a
         Route Target that is not in this set, this Route Target is
         immediately added to the set.  On the other hand, when the
         route reflector no longer has any routes with a particular
         Route Target that is in the set, the route reflector should
         delay (by a few hours) the deletion of this Route Target from
         the set.

         The route reflector uses this set to form the inbound route
         filters that it applies to routes received from other route
         reflectors.  The route reflector may also use ORFs to install
         the appropriate outbound route filtering on other route
         reflectors.  Just like with the first approach, a route
         reflector should accept ORFs from other route reflectors.  To
         accomplish this, a route reflector advertises ORF capability to
         other route reflectors.

         When the route reflector changes the set, it should immediately
         change its inbound route filtering.  In addition, if the route
         reflector uses ORFs, then the ORFs have to be immediately
         changed to reflect the changes in the set.  If the route
         reflector doesn't use ORFs, and a new Route Target is added to

         the set, the route reflector, after changing its inbound route
         filtering, must issue BGP Refresh to other route reflectors.

         The delay of "a few hours" mentioned above allows a route
         reflector to hold onto routes with a given RT, even after it
         loses the last of its clients that are interested in such
         routes.  This protects against the need to reacquire all such
         routes if the clients' "disappearance" is only temporary.

         With this procedure, VPN Join and Prune operations are also
         non-disruptive.

         Note that this technique will not work properly if some client
         PE has a VRF with an import Route Target that is not one of its
         export Route Targets.

   In these procedures, a PE router which attaches to a particular VPN
   "auto-discovers" the other PEs that attach to the same VPN.  When a
   new PE router is added, or when an existing PE router attaches to a
   new VPN, no reconfiguration of other PE routers is needed.

   Just as there is no one PE router that needs to know all the VPN-IPv4
   routes supported over the backbone, these distribution rules ensure
   that there is no one Route Reflector (RR) that needs to know all the
   VPN-IPv4 routes supported over the backbone.  As a result, the total
   number of such routes that can be supported over the backbone is not
   bounded by the capacity of any single device, and therefore can
   increase virtually without bound.

4.3.4. How VPN-IPv4 NLRI Is Carried in BGP

   The BGP Multiprotocol Extensions [BGP-MP] are used to encode the
   NLRI.  If the Address Family Identifier (AFI) field is set to 1, and
   the Subsequent Address Family Identifier (SAFI) field is set to 128,
   the NLRI is an MPLS-labeled VPN-IPv4 address.  AFI 1 is used since
   the network layer protocol associated with the NLRI is still IP.
   Note that this VPN architecture does not require the capability to
   distribute unlabeled VPN-IPv4 addresses.

   In order for two BGP speakers to exchange labeled VPN-IPv4 NLRI, they
   must use BGP Capabilities Advertisement to ensure that they both are
   capable of properly processing such NLRI.  This is done as specified
   in [BGP-MP], by using capability code 1 (multiprotocol BGP), with an
   AFI of 1 and an SAFI of 128.

   The labeled VPN-IPv4 NLRI itself is encoded as specified in
   [MPLS-BGP], where the prefix consists of an 8-byte RD followed by an
   IPv4 prefix.

4.3.5. Building VPNs Using Route Targets

   By setting up the Import Targets and Export Targets properly, one can
   construct different kinds of VPNs.

   Suppose it is desired to create a fully meshed closed user group,
   i.e., a set of sites where each can send traffic directly to the
   other, but traffic cannot be sent to or received from other sites.
   Then each site is associated with a VRF, a single Route Target
   attribute is chosen, that Route Target is assigned to each VRF as
   both the Import Target and the Export Target, and that Route Target
   is not assigned to any other VRFs as either the Import Target or the
   Export Target.

   Alternatively, suppose one desired, for whatever reason, to create a
   "hub and spoke" kind of VPN.  This could be done by the use of two
   Route Target values, one meaning "Hub" and one meaning "Spoke".  At
   the VRFs attached to the hub sites, "Hub" is the Export Target and

   "Spoke" is the Import Target.  At the VRFs attached to the spoke
   site, "Hub" is the Import Target and "Spoke" is the Export Target.

   Thus, the methods for controlling the distribution of routing
   information among various sets of sites are very flexible, which in
   turn provides great flexibility in constructing VPNs.

4.3.6. Route Distribution Among VRFs in a Single PE

   It is possible to distribute routes from one VRF to another, even if
   both VRFs are in the same PE, even though in this case one cannot say
   that the route has been distributed by BGP.  Nevertheless, the
   decision to distribute a particular route from one VRF to another
   within a single PE is the same decision that would be made if the
   VRFs were on different PEs.  That is, it depends on the Route Target
   attribute that is assigned to the route (or would be assigned if the
   route were distributed by BGP), and the import target of the second
   VRF.

5. Forwarding

   If the intermediate routers in the backbone do not have any
   information about the routes to the VPNs, how are packets forwarded
   from one VPN site to another?

   When a PE receives an IP packet from a CE device, it chooses a
   particular VRF in which to look up the packet's destination address.
   This choice is based on the packet's ingress attachment circuit.

   Assume that a match is found.  As a result we learn the packet's
   "next hop".

   If the packet's next hop is reached directly over a VRF attachment
   circuit from this PE (i.e., the packet's egress attachment circuit is
   on the same PE as its ingress attachment circuit), then the packet is
   sent on the egress attachment circuit, and no MPLS labels are pushed
   onto the packet's label stack.

   If the ingress and egress attachment circuits are on the same PE, but
   are associated with different VRFs, and if the route that best
   matches the destination address in the ingress attachment circuit's
   VRF is an aggregate of several routes in the egress attachment
   circuit's VRF, it may be necessary to look up the packet's
   destination address in the egress VRF as well.

   If the packet's next hop is NOT reached through a VRF attachment
   circuit, then the packet must travel at least one hop through the
   backbone.  The packet thus has a "BGP Next Hop", and the BGP Next Hop
   will have assigned an MPLS label for the route that best matches the
   packet's destination address.  Call this label the "VPN route label".
   The IP packet is turned into an MPLS packet with the VPN route label
   as the sole label on the label stack.

   The packet must then be tunneled to the BGP Next Hop.

   If the backbone supports MPLS, this is done as follows:

     - The PE routers (and any Autonomous System border routers) that
       redistribute VPN-IPv4 addresses need to insert /32 address
       prefixes for themselves into the IGP routing tables of the
       backbone.  This enables MPLS, at each node in the backbone
       network, to assign a label corresponding to the route to each PE
       router.  To ensure interoperability among different
       implementations, it is required to support LDP for setting up the
       label switched paths across the backbone.  However, other methods
       of setting up these label switched paths are also possible.
       (Some of these other methods may not require the presence of the
       /32 address prefixes in the IGP.)

     - If there are any traffic engineering tunnels to the BGP next hop,
       and if one or more of those is available for use by the packet in
       question, one of these tunnels is chosen.  This tunnel will be
       associated with an MPLS label, the "tunnel label".  The tunnel
       label gets pushed on the MPLS label stack, and the packet is
       forwarded to the tunnel's next hop.

     - Otherwise,

         * The packet will have an "IGP Next Hop", which is the next hop
           along the IGP route to the BGP Next Hop.

         * If the BGP Next Hop and the IGP Next Hop are the same, and if
           penultimate hop popping is used, the packet is then sent to
           the IGP Next Hop, carrying only the VPN route label.

         * Otherwise, the IGP Next Hop will have assigned a label for
           the route that best matches the address of the BGP Next Hop.
           Call this the "tunnel label".  The tunnel label gets pushed
           on as the packet's top label.  The packet is then forwarded
           to the IGP Next Hop.

     - MPLS will then carry the packet across the backbone to the BGP
       Next Hop, where the VPN label will be examined.

   If the backbone does not support MPLS, the MPLS packet carrying only
   the VPN route label may be tunneled to the BGP Next Hop using the
   techniques of [MPLS-in-IP-GRE].  When the packet emerges from the
   tunnel, it will be at the BGP Next Hop, where the VPN route label
   will be examined.

   At the BGP Next Hop, the treatment of the packet depends on the VPN
   route label (see Section 4.3.2).  In many cases, the PE will be able
   to determine, from this label, the attachment circuit over which the
   packet should be transmitted (to a CE device), as well as the proper
   data link layer header for that interface.  In other cases, the PE
   may only be able to determine that the packet's destination address
   needs to be looked up in a particular VRF before being forwarded to a
   CE device.  There are also intermediate cases in which the VPN route
   label may determine the packet's egress attachment circuit, but a
   lookup (e.g., ARP) still needs to be done in order to determine the
   packet's data link header on that attachment circuit.

   Information in the MPLS header itself, and/or information associated
   with the label, may also be used to provide QoS on the interface to
   the CE.

   In any event, if the packet was an unlabeled IP packet when it
   arrived at its ingress PE, it will again be an unlabeled packet when
   it leaves its egress PE.

   The fact that packets with VPN route labels are tunneled through the
   backbone is what makes it possible to keep all the VPN routes out of
   the P routers.  This is crucial to ensuring the scalability of the

   scheme.  The backbone does not even need to have routes to the CEs,
   only to the PEs.

   With respect to the tunnels, it is worth noting that this
   specification:

     - DOES NOT require that the tunnels be point-to-point; multipoint-
       to-point can be used;

     - DOES NOT require that there be any explicit setup of the tunnels,
       either via signaling or via manual configuration;

     - DOES NOT require that there be any tunnel-specific signaling;

     - DOES NOT require that there be any tunnel-specific state in the P
       or PE routers, beyond what is necessary to maintain the routing
       information and (if used) the MPLS label information.

   Of course, this specification is compatible with the use of point-
   to-point tunnels that must be explicitly configured and/or signaled,
   and in some situations there may be reasons for using such tunnels.

   The considerations that are relevant to choosing a particular
   tunneling technology are outside the scope of this specification.

6. Maintaining Proper Isolation of VPNs

   To maintain proper isolation of one VPN from another, it is important
   that no router in the backbone accept a tunneled packet from outside
   the backbone, unless it is sure that both endpoints of that tunnel
   are outside the backbone.

   If MPLS is being used as the tunneling technology, this means that a
   router in the backbone MUST NOT accept a labeled packet from any
   adjacent non-backbone device unless the following two conditions
   hold:

      1. the label at the top of the label stack was actually
         distributed by that backbone router to that non-backbone
         device, and

      2. the backbone router can determine that use of that label will
         cause the packet to leave the backbone before any labels lower
         in the stack will be inspected, and before the IP header will
         be inspected.

   The first condition ensure that any labeled packets received from
   non-backbone routers have a legitimate and properly assigned label at

   the top of the label stack.  The second condition ensures that the
   backbone routers will never look below that top label.  Of course,
   the simplest way to meet these two conditions is just to have the
   backbone devices refuse to accept labeled packets from non-backbone
   devices.

   If MPLS is not being used as the tunneling technology, then filtering
   must be done to ensure that an MPLS-in-IP or MPLS-in-GRE packet can
   be accepted into the backbone only if the packet's IP destination
   address will cause it to be sent outside the backbone.

7. How PEs Learn Routes from CEs

   The PE routers that attach to a particular VPN need to know, for each
   attachment circuit leading to that VPN, which of the VPN's addresses
   should be reached over that attachment circuit.

   The PE translates these addresses into VPN-IPv4 addresses, using a
   configured RD.  The PE then treats these VPN-IPv4 routes as input to
   BGP.  Routes from a VPN site are NOT leaked into the backbone's IGP.

   Exactly which PE/CE route distribution techniques are possible
   depends on whether or not a particular CE is in a "transit VPN".  A
   "transit VPN" is one that contains a router that receives routes from
   a "third party" (i.e., from a router that is not in the VPN, but is
   not a PE router) and that redistributes those routes to a PE router.
   A VPN that is not a transit VPN is a "stub VPN".  The vast majority
   of VPNs, including just about all corporate enterprise networks,
   would be expected to be "stubs" in this sense.

   The possible PE/CE distribution techniques are:

      1. Static routing (i.e., configuration) may be used. (This is
         likely to be useful only in stub VPNs.)

      2. PE and CE routers may be Routing Information Protocol (RIP)
         [RIP] peers, and the CE may use RIP to tell the PE router the
         set of address prefixes that are reachable at the CE router's
         site.  When RIP is configured in the CE, care must be taken to
         ensure that address prefixes from other sites (i.e., address
         prefixes learned by the CE router from the PE router) are never
         advertised to the PE.  More precisely:  if a PE router, say,
         PE1, receives a VPN-IPv4 route R1, and as a result distributes
         an IPv4 route R2 to a CE, then R2 must not be distributed back
         from that CE's site to a PE router, say, PE2, (where PE1 and
         PE2 may be the same router or different routers), unless PE2
         maps R2 to a VPN-IPv4 route that is different than (i.e.,
         contains a different RD than) R1.

      3. The PE and CE routers may be OSPF peers.  A PE router that is
         an OSPF peer of a CE router appears, to the CE router, to be an
         area 0 router.  If a PE router is an OSPF peer of CE routers
         that are in distinct VPNs, the PE must of course be running
         multiple instances of OSPF.

         IPv4 routes that the PE learns from the CE via OSPF are
         redistributed into BGP as VPN-IPv4 routes.  Extended Community
         attributes are used to carry, along with the route, all the
         information needed to enable the route to be distributed to
         other CE routers in the VPN in the proper type of OSPF Link
         State Advertisement (LSA).  OSPF route tagging is used to
         ensure that routes received from the MPLS/BGP backbone are not
         sent back into the backbone.

         Specification of the complete set of procedures for the use of
         OSPF between PE and CE can be found in [VPN-OSPF] and
         [OSPF-2547-DNBIT].

      4. The PE and CE routers may be BGP peers, and the CE router may
         use BGP (in particular, EBGP to tell the PE router the set of
         address prefixes that are at the CE router's site. (This
         technique can be used in stub VPNs or transit VPNs.)

         This technique has a number of advantages over the others:

            a) Unlike the IGP alternatives, this does not require the PE
               to run multiple routing algorithm instances in order to
               talk to multiple CEs.

            b) BGP is explicitly designed for just this function:
               passing routing information between systems run by
               different administrations.

            c) If the site contains "BGP backdoors", i.e., routers with
               BGP connections to routers other than PE routers, this
               procedure will work correctly in all circumstances.  The
               other procedures may or may not work, depending on the
               precise circumstances.

            d) Use of BGP makes it easy for the CE to pass attributes of
               the routes to the PE.  A complete specification of the
               set of attributes and their use is outside the scope of
               this document.  However, some examples of the way this
               may be used are the following:

                 - The CE may suggest a particular Route Target for each
                   route, from among the Route Targets that the PE is
                   authorized to attach to the route.  The PE would then
                   attach only the suggested Route Target, rather than
                   the full set.  This gives the CE administrator some
                   dynamic control of the distribution of routes from
                   the CE.

                 - Additional types of Extended Community attributes may
                   be defined, where the intention is to have those
                   attributes passed transparently (i.e., without being
                   changed by the PE routers) from CE to CE.  This would
                   allow CE administrators to implement additional route
                   filtering, beyond that which is done by the PEs.
                   This additional filtering would not require
                   coordination with the SP.

         On the other hand, using BGP may be something new for the CE
         administrators.

         If a site is not in a transit VPN, note that it need not have a
         unique Autonomous System Number (ASN).  Every CE whose site is
         not in a transit VPN can use the same ASN.  This can be chosen
         from the private ASN space, and it will be stripped out by the
         PE.  Routing loops are prevented by use of the Site of Origin
         attribute (see below).

         What if a set of sites constitutes a transit VPN?  This will
         generally be the case only if the VPN is itself an Internet
         Service Provider's (ISP's) network, where the ISP is itself
         buying backbone services from another SP.  The latter SP may be
         called a "carrier's carrier".  In this case, the best way to
         provide the VPN is to have the CE routers support MPLS, and to
         use the technique described in Section 9.

   When we do not need to distinguish among the different ways in which
   a PE can be informed of the address prefixes that exist at a given
   site, we will simply say that the PE has "learned" the routes from
   that site.  This includes the case where the PE has been manually
   configured with the routes.

   Before a PE can redistribute a VPN-IPv4 route learned from a site, it
   must assign a Route Target attribute (see Section 4.3.1) to the
   route, and it may assign a Site of Origin attribute to the route.

   The Site of Origin attribute, if used, is encoded as a Route Origin
   Extended Community [BGP-EXTCOMM].  The purpose of this attribute is
   to uniquely identify the set of routes learned from a particular

   site.  This attribute is needed in some cases to ensure that a route
   learned from a particular site via a particular PE/CE connection is
   not distributed back to the site through a different PE/CE
   connection.  It is particularly useful if BGP is being used as the
   PE/CE protocol, but different sites have not been assigned distinct
   ASNs.

8. How CEs Learn Routes from PEs

   In this section, we assume that the CE device is a router.

   If the PE places a particular route in the VRF it uses to route
   packets received from a particular CE, then in general, the PE may
   distribute that route to the CE.  Of course, the PE may distribute
   that route to the CE only if this is permitted by the rules of the
   PE/CE protocol.  (For example, if a particular PE/CE protocol has
   "split horizon", certain routes in the VRF cannot be redistributed
   back to the CE.)  We add one more restriction on the distribution of
   routes from PE to CE: if a route's Site of Origin attribute
   identifies a particular site, that route must never be redistributed
   to any CE at that site.

   In most cases, however, it will be sufficient for the PE to simply
   distribute the default route to the CE.  (In some cases, it may even
   be sufficient for the CE to be configured with a default route
   pointing to the PE.)  This will generally work at any site that does
   not itself need to distribute the default route to other sites.
   (E.g., if one site in a corporate VPN has the corporation's access to
   the Internet, that site might need to have default distributed to the
   other site, but one could not distribute default to that site
   itself.)

   Whatever procedure is used to distribute routes from CE to PE will
   also be used to distribute routes from PE to CE.

9. Carriers' Carriers

   Sometimes a VPN may actually be the network of an ISP, with its own
   peering and routing policies.  Sometimes a VPN may be the network of
   an SP that is offering VPN services in turn to its own customers.
   VPNs like these can also obtain backbone service from another SP, the
   "carrier's carrier", using essentially the same methods described in
   this document.  However, it is necessary in these cases that the CE
   routers support MPLS.  In particular:

     - The CE routers should distribute to the PE routers ONLY those
       routes that are internal to the VPN.  This allows the VPN to be
       handled as a stub VPN.

     - The CE routers should support MPLS, in that they should be able
       to receive labels from the PE routers, and send labeled packets
       to the PE routers.  They do not need to distribute labels of
       their own, though.

     - The PE routers should distribute, to the CE routers, labels for
       the routes they distribute to the CE routers.

       The PE must not distribute the same label to two different CEs
       unless one of the following conditions holds:

         * The two CEs are associated with exactly the same set of VRFs;

         * The PE maintains a different Incoming Label Map ([MPLS-ARCH])
           for each CE.

       Further, when the PE receives a labeled packet from a CE, it must
       verify that the top label is one that was distributed to that CE.

     - Routers at the different sites should establish BGP connections
       among themselves for the purpose of exchanging external routes
       (i.e., routes that lead outside of the VPN).

     - All the external routes must be known to the CE routers.

   Then when a CE router looks up a packet's destination address, the
   routing lookup will resolve to an internal address, usually the
   address of the packet's BGP next hop.  The CE labels the packet
   appropriately and sends the packet to the PE.  The PE, rather than
   looking up the packet's IP destination address in a VRF, uses the
   packet's top MPLS label to select the BGP next hop.  As a result, if
   the BGP next hop is more than one hop away, the top label will be
   replaced by two labels, a tunnel label and a VPN route label.  If the
   BGP next hop is one hop away, the top label may be replaced by just
   the VPN route label.  If the ingress PE is also the egress PE, the
   top label will just be popped.  When the packet is sent from its
   egress PE to a CE, the packet will have one fewer MPLS labels than it
   had when it was first received by its ingress PE.

   In the above procedure, the CE routers are the only routers in the
   VPN that need to support MPLS.  If, on the other hand, all the
   routers at a particular VPN site support MPLS, then it is no longer
   required that the CE routers know all the external routes.  All that
   is required is that the external routes be known to whatever routers
   are responsible for putting the label stack on a hitherto unlabeled
   packet and that there be label switched path that leads from those
   routers to their BGP peers at other sites.  In this case, for each

internal route that a CE router distributes to a PE router, it must
also distribute a label.

10. Multi-AS Backbones

   What if two sites of a VPN are connected to different Autonomous
   Systems (e.g., because the sites are connected to different SPs)?
   The PE routers attached to that VPN will then not be able to maintain
   IBGP connections with each other, or with a common route reflector.
   Rather, there needs to be some way to use EBGP to distribute VPN-IPv4
   addresses.

   There are a number of different ways of handling this case, which we
   present in order of increasing scalability.

      a) VRF-to-VRF connections at the AS (Autonomous System) border
         routers.

         In this procedure, a PE router in one AS attaches directly to a
         PE router in another.  The two PE routers will be attached by
         multiple sub-interfaces, at least one for each of the VPNs
         whose routes need to be passed from AS to AS.  Each PE will
         treat the other as if it were a CE router.  That is, the PEs
         associate each such sub-interface with a VRF, and use EBGP to
         distribute unlabeled IPv4 addresses to each other.

         This is a procedure that "just works", and that does not
         require MPLS at the border between ASes.  However, it does not
         scale as well as the other procedures discussed below.

      b) EBGP redistribution of labeled VPN-IPv4 routes from AS to
         neighboring AS.

         In this procedure, the PE routers use IBGP to redistribute
         labeled VPN-IPv4 routes either to an Autonomous System Border
         Router (ASBR), or to a route reflector of which an ASBR is a
         client.  The ASBR then uses EBGP to redistribute those labeled
         VPN-IPv4 routes to an ASBR in another AS, which in turn
         distributes them to the PE routers in that AS, or perhaps to
         another ASBR which in turn distributes them, and so on.

         When using this procedure, VPN-IPv4 routes should only be
         accepted on EBGP connections at private peering points, as part
         of a trusted arrangement between SPs.  VPN-IPv4 routes should
         neither be distributed to nor accepted from the public
         Internet, or from any BGP peers that are not trusted.  An ASBR
         should never accept a labeled packet from an EBGP peer unless
         it has actually distributed the top label to that peer.

         If there are many VPNs having sites attached to different
         Autonomous Systems, there does not need to be a single ASBR
         between those two ASes that holds all the routes for all the
         VPNs; there can be multiple ASBRs, each of which holds only the
         routes for a particular subset of the VPNs.

         This procedure requires that there be a label switched path
         leading from a packet's ingress PE to its egress PE.  Hence the
         appropriate trust relationships must exist between and among
         the set of ASes along the path.  Also, there must be agreement
         among the set of SPs as to which border routers need to receive
         routes with which Route Targets.

      c) Multi-hop EBGP redistribution of labeled VPN-IPv4 routes
         between source and destination ASes, with EBGP redistribution
         of labeled IPv4 routes from AS to neighboring AS.

         In this procedure, VPN-IPv4 routes are neither maintained nor
         distributed by the ASBRs.  An ASBR must maintain labeled IPv4
         /32 routes to the PE routers within its AS.  It uses EBGP to
         distribute these routes to other ASes.  ASBRs in any transit
         ASes will also have to use EBGP to pass along the labeled /32
         routes.  This results in the creation of a label switched path
         from the ingress PE router to the egress PE router.  Now PE
         routers in different ASes can establish multi-hop EBGP
         connections to each other, and can exchange VPN-IPv4 routes
         over those connections.

         If the /32 routes for the PE routers are made known to the P
         routers of each AS, everything works normally.  If the /32
         routes for the PE routers are NOT made known to the P routers
         (other than the ASBRs), then this procedure requires a packet's
         ingress PE to put a three-label stack on it.  The bottom label
         is assigned by the egress PE, corresponding to the packet's
         destination address in a particular VRF.  The middle label is
         assigned by the ASBR, corresponding to the /32 route to the
         egress PE.  The top label is assigned by the ingress PE's IGP
         Next Hop, corresponding to the /32 route to the ASBR.

         To improve scalability, one can have the multi-hop EBGP
         connections exist only between a route reflector in one AS and
         a route reflector in another.  (However, when the route
         reflectors distribute routes over this connection, they do not
         modify the BGP next hop attribute of the routes.)  The actual
         PE routers would then only have IBGP connections to the route
         reflectors in their own AS.

         This procedure is very similar to the "carrier's carrier"
         procedures described in Section 9.  Like the previous
         procedure, it requires that there be a label switched path
         leading from a packet's ingress PE to its egress PE.

11. Accessing the Internet from a VPN

   Many VPN sites will need to be able to access the public Internet, as
   well as to access other VPN sites.  The following describes some of
   the alternative ways of doing this.

      1. In some VPNs, one or more of the sites will obtain Internet
         access by means of an "Internet gateway" (perhaps a firewall)
         attached to a non-VRF interface to an ISP.  The ISP may or may
         not be the same organization as the SP that is providing the
         VPN service.  Traffic to/from the Internet gateway would then
         be routed according to the PE router's default forwarding
         table.

         In this case, the sites that have Internet access may be
         distributing a default route to their PEs, which in turn
         redistribute it to other PEs and hence into other sites of the
         VPN.  This provides Internet access for all of the VPN's sites.

         In order to properly handle traffic from the Internet, the ISP
         must distribute, to the Internet, routes leading to addresses
         that are within the VPN.  This is completely independent of any
         of the route distribution procedures described in this
         document.  The internal structure of the VPN will in general
         not be visible from the Internet; such routes would simply lead
         to the non-VRF interface that attaches to the VPN's Internet
         gateway.

         In this model, there is no exchange of routes between a PE
         router's default forwarding table and any of its VRFs.  VPN
         route distribution procedures and Internet route distribution
         procedures are completely independent.

         Note that although some sites of the VPN use a VRF interface to
         communicate with the Internet, ultimately all packets to/from
         the Internet traverse a non-VRF interface before
         leaving/entering the VPN, so we refer to this as "non-VRF
         Internet access".

         Note that the PE router to which the non-VRF interface attaches
         does not necessarily need to maintain all the Internet routes
         in its default forwarding table.  The default forwarding table
         could have as few as one route, "default", which leads to

         another router (probably an adjacent one) that has the Internet
         routes.  A variation of this scheme is to tunnel packets
         received over the non-VRF interface from the PE router to
         another router, where this other router maintains the full set
         of Internet routes.

      2. Some VPNs may obtain Internet access via a VRF interface ("VRF
         Internet access").  If a packet is received by a PE over a VRF
         interface, and if the packet's destination address does not
         match any route in the VRF, then it may be matched against the
         PE's default forwarding table.  If a match is made there, the
         packet can be forwarded natively through the backbone to the
         Internet, instead of being forwarded by MPLS.

         In order for traffic to flow natively in the opposite direction
         (from Internet to VRF interface), some of the routes from the
         VRF must be exported to the Internet forwarding table.
         Needless to say, any such routes must correspond to globally
         unique addresses.

         In this scheme, the default forwarding table might have the
         full set of Internet routes, or it might have as little as a
         single default route leading to another router that does have
         the full set of Internet routes in its default forwarding
         table.

      3. Suppose the PE has the capability to store "non-VPN routes" in
         a VRF.  If a packet's destination address matches a "non-VPN
         route", then the packet is transmitted natively, rather than
         being transmitted via MPLS.  If the VRF contains a non-VPN
         default route, all packets for the public Internet will match
         it, and be forwarded natively to the default route's next hop.
         At that next hop, the packets' destination addresses will be
         looked up in the default forwarding table, and may match more
         specific routes.

         This technique would only be available if none of the CE
         routers is distributing a default route.

      4. It is also possible to obtain Internet access via a VRF
         interface by having the VRF contain the Internet routes.
         Compared with model 2, this eliminates the second lookup, but
         it has the disadvantage of requiring the Internet routes to be
         replicated in each such VRF.

         If this technique is used, the SP may want to make its
         interface to the Internet be a VRF interface, and to use the

         techniques of Section 4 to distribute Internet routes, as VPN-
         IPv4 routes, to other VRFs.

   It should be clearly understood that by default, there is no exchange
   of routes between a VRF and the default forwarding table.  This is
   done ONLY upon agreement between a customer and an SP, and only if it
   suits the customer's policies.

12. Management VPNs

   This specification does not require that the sub-interface connecting
   a PE router and a CE router be a "numbered" interface.  If it is a
   numbered interface, this specification allows the addresses assigned
   to the interface to come from either the address space of the VPN or
   the address space of the SP.

   If a CE router is being managed by the Service Provider, then the
   Service Provider will likely have a network management system that
   needs to be able to communicate with the CE router.  In this case,
   the addresses assigned to the sub-interface connecting the CE and PE
   routers should come from the SP's address space, and should be unique
   within that space.  The network management system should itself
   connect to a PE router (more precisely, be at a site that connects to
   a PE router) via a VRF interface.  The address of the network
   management system will be exported to all VRFs that are associated
   with interfaces to CE routers that are managed by the SP.  The
   addresses of the CE routers will be exported to the VRF associated
   with the network management system, but not to any other VRFs.

   This allows communication between the CE and network management
   system, but does not allow any undesired communication to or among
   the CE routers.

   One way to ensure that the proper route import/exports are done is to
   use two Route Targets; call them T1 and T2.  If a particular VRF
   interface attaches to a CE router that is managed by the SP, then
   that VRF is configured to:

     - import routes that have T1 attached to them, and

     - attach T2 to addresses assigned to each end of its VRF
       interfaces.

   If a particular VRF interface attaches to the SP's network management
   system, then that VRF is configured to attach T1 to the address of
   that system, and to import routes that have T2 attached to them.

13. Security Considerations

13.1. Data Plane

   By security in the "data plane", we mean protection against the
   following possibilities:

     - Packets from within a VPN travel to a site outside the VPN, other
       than in a manner consistent with the policies of the VPN.

     - Packets from outside a VPN enter one of the VPN's sites, other
       than in a manner consistent with the policies of the VPN.

   Under the following conditions:

      1. a backbone router does not accept labeled packets over a
         particular data link, unless it is known that that data link
         attaches only to trusted systems, or unless it is known that
         such packets will leave the backbone before the IP header or
         any labels lower in the stack will be inspected, and

      2. labeled VPN-IPv4 routes are not accepted from untrusted or
         unreliable routing peers,

      3. no successful attacks have been mounted on the control plane,

   the data plane security provided by this architecture is virtually
   identical to that provided to VPNs by Frame Relay or ATM backbones.
   If the devices under the control of the SP are properly configured,
   data will not enter or leave a VPN unless authorized to do so.

   Condition 1 above can be stated more precisely.  One should discard a
   labeled packet received from a particular neighbor unless one of the
   following two conditions holds:

     - the packet's top label has a label value that the receiving
       system has distributed to that neighbor, or

     - the packet's top label has a label value that the receiving
       system has distributed to a system beyond that neighbor (i.e.,
       when it is known that the path from the system to which the label
       was distributed to the receiving system may be via that
       neighbor).

   Condition 2 above is of most interest in the case of inter-provider
   VPNs (see Section 10).  For inter-provider VPNs constructed according
   to scheme b) of Section 10, condition 2 is easily checked.  (The
   issue of security when scheme (c) of Section 10 is used is for
   further study.)

   It is worth noting that the use of MPLS makes it much simpler to
   provide data plane security than might be possible if one attempted
   to use some form of IP tunneling in place of the MPLS outer label.
   It is a simple matter to have one's border routers refuse to accept a
   labeled packet unless the first of the above conditions applies to
   it.  It is rather more difficult to configure a router to refuse to
   accept an IP packet if that packet is an IP tunneled packet whose
   destination address is that of a PE router; certainly, this is not
   impossible to do, but it has both management and performance
   implications.

   MPLS-in-IP and MPLS-in-GRE tunneling are specified in
   [MPLS-in-IP-GRE].  If it is desired to use such tunnels to carry VPN
   packets, then the security considerations described in Section 8 of
   that document must be fully understood.  Any implementation of
   BGP/MPLS IP VPNs that allows VPN packets to be tunneled as described
   in that document MUST contain an implementation of IPsec that can be
   used as therein described.  If the tunnel is not secured by IPsec,
   then the technique of IP address filtering at the border routers,
   described in Section 8.2 of that document, is the only means of
   ensuring that a packet that exits the tunnel at a particular egress
   PE was actually placed in the tunnel by the proper tunnel head node
   (i.e., that the packet does not have a spoofed source address).
   Since border routers frequently filter only source addresses, packet
   filtering may not be effective unless the egress PE can check the IP
   source address of any tunneled packet it receives, and compare it to
   a list of IP addresses that are valid tunnel head addresses.  Any
   implementation that allows MPLS-in-IP and/or MPLS-in-GRE tunneling to
   be used without IPsec MUST allow the egress PE to validate in this
   manner the IP source address of any tunneled packet that it receives.

   In the case where a number of CE routers attach to a PE router via a
   LAN interface, to ensure proper security, one of the following
   conditions must hold:

      1. All the CE routers on the LAN belong to the same VPN, or

      2. A trusted and secured LAN switch divides the LAN into multiple
         VLANs, with each VLAN containing only systems of a single VPN;
         in this case, the switch will attach the appropriate VLAN tag
         to any packet before forwarding it to the PE router.

   Cryptographic privacy is not provided by this architecture, nor by
   Frame Relay or ATM VPNs.  These architectures are all compatible with
   the use of cryptography on a CE-CE basis, if that is desired.

   The use of cryptography on a PE-PE basis is for further study.

13.2. Control Plane

   The data plane security of the previous section depends on the
   security of the control plane.  To ensure security, neither BGP nor
   LDP connections should be made with untrusted peers.  The TCP/IP MD5
   authentication option [TCP-MD5] should be used with both these
   protocols.  The routing protocol within the SP's network should also
   be secured in a similar manner.

13.3. Security of P and PE Devices

   If the physical security of these devices is compromised, data plane
   security may also be compromised.

   The usual steps should be taken to ensure that IP traffic from the
   public Internet cannot be used to modify the configuration of these
   devices, or to mount Denial of Service attacks on them.

14. Quality of Service

   Although not the focus of this paper, Quality of Service is a key
   component of any VPN service.  In MPLS/BGP VPNs, existing L3 QoS
   capabilities can be applied to labeled packets through the use of the
   "experimental" bits in the shim header [MPLS-ENCAPS], or, where ATM
   is used as the backbone, through the use of ATM QoS capabilities.
   The traffic engineering work discussed in [MPLS-RSVP] is also
   directly applicable to MPLS/BGP VPNs.  Traffic engineering could even
   be used to establish label switched paths with particular QoS
   characteristics between particular pairs of sites, if that is
   desirable.  Where an MPLS/BGP VPN spans multiple SPs, the
   architecture described in [PASTE] may be useful.  An SP may apply
   either intserv (Integrated Services) or diffserv (Differentiated
   Services) capabilities to a particular VPN, as appropriate.

15. Scalability

   We have discussed scalability issues throughout this paper.  In this
   section, we briefly summarize the main characteristics of our model
   with respect to scalability.

   The Service Provider backbone network consists of (a) PE routers, (b)
   BGP Route Reflectors, (c) P routers (that are neither PE routers nor
   Route Reflectors), and, in the case of multi-provider VPNs, (d)
   ASBRs.

   P routers do not maintain any VPN routes.  In order to properly
   forward VPN traffic, the P routers need only maintain routes to the
   PE routers and the ASBRs.  The use of two levels of labeling is what
   makes it possible to keep the VPN routes out of the P routers.

   A PE router maintains VPN routes, but only for those VPNs to which it
   is directly attached.

   Route reflectors can be partitioned among VPNs so that each partition
   carries routes for only a subset of the VPNs supported by the Service
   Provider.  Thus, no single route reflector is required to maintain
   routes for all VPNs.

   For inter-provider VPNs, if the ASBRs maintain and distribute VPN-
   IPv4 routes, then the ASBRs can be partitioned among VPNs in a
   similar manner, with the result that no single ASBR is required to
   maintain routes for all the inter-provider VPNs.  If multi-hop EBGP
   is used, then the ASBRs need not maintain and distribute VPN-IPv4
   routes at all.

   As a result, no single component within the Service Provider network
   has to maintain all the routes for all the VPNs.  So the total
   capacity of the network to support increasing numbers of VPNs is not
   limited by the capacity of any individual component.

16. IANA Considerations

   The Internet Assigned Numbers Authority (IANA) has created a new
   registry for the "Route Distinguisher Type Field" (see Section 4.2).
   This is a two-byte field.  Types 0, 1, and 2 are defined by this
   document.  Additional Route Distinguisher Type Field values with a
   high-order bit of 0 may be allocated by IANA on a "First Come, First
   Served" basis [IANA].  Values with a high-order bit of 1 may be
   allocated by IANA based on "IETF consensus" [IANA].

   This document specifies (see Section 4.3.4) the use of the BGP
   Address Family Identifier (AFI) value 1, along with the BGP
   Subsequent Address Family Identifier (SAFI) value 128, to represent
   the address family "VPN-IPv4 Labeled Addresses", which is defined in
   this document.

   The use of AFI value 1 for IP is as currently specified in the IANA
   registry "Address Family Identifier", so IANA need take no action
   with respect to it.

   The SAFI value 128 was originally specified as "Private Use" in the
   IANA "Subsequent Address Family Identifier" registry.  IANA has
   changed the SAFI value 128 from "private use" to "MPLS-labeled VPN
   address".

17. Acknowledgements

   The full list of contributors can be found in Section 18.

   Significant contributions to this work have also been made by Ravi
   Chandra, Dan Tappan, and Bob Thomas.

   We also wish to thank Shantam Biswas for his review and
   contributions.

18. Contributors

   Tony Bogovic
   Telcordia Technologies
   445 South Street, Room 1A264B
   Morristown, NJ 07960

   EMail: tjb@research.telcordia.com

   Stephen John Brannon
   Swisscom AG
   Postfach 1570
   CH-8301
   Glattzentrum (Zuerich), Switzerland

   EMail: stephen.brannon@swisscom.com

   Marco Carugi
   Nortel Networks S.A.
   Parc d'activites de Magny-Les Jeunes Bois  CHATEAUFORT
   78928 YVELINES Cedex 9 - FRANCE

   EMail: marco.carugi@nortelnetworks.com

   Christopher J. Chase
   AT&T
   200 Laurel Ave
   Middletown, NJ 07748
   USA

   EMail: chase@att.com

   Ting Wo Chung
   Bell Nexxia
   181 Bay Street
   Suite 350
   Toronto, Ontario
   M5J2T3

   EMail: ting_wo.chung@bellnexxia.com

   Eric Dean

   Jeremy De Clercq
   Alcatel Network Strategy Group
   Francis Wellesplein 1
   2018 Antwerp, Belgium

   EMail: jeremy.de_clercq@alcatel.be

   Luyuan Fang
   AT&T
   IP Backbone Architecture
   200 Laurel Ave.
   Middletown, NJ 07748

   EMail: luyuanfang@att.com

   Paul Hitchen
   BT
   BT Adastral Park
   Martlesham Heath,
   Ipswich IP5 3RE
   UK

   EMail: paul.hitchen@bt.com

   Manoj Leelanivas
   Juniper Networks, Inc.
   385 Ravendale Drive
   Mountain View, CA 94043 USA

   EMail: manoj@juniper.net

   Dave Marshall
   Worldcom
   901 International Parkway
   Richardson, Texas 75081

   EMail: dave.marshall@wcom.com

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112

   EMail: lmartini@cisco.com

   Monique Jeanne Morrow
   Cisco Systems, Inc.
   Glatt-com, 2nd floor
   CH-8301
   Glattzentrum, Switzerland

   EMail: mmorrow@cisco.com

   Ravichander Vaidyanathan
   Telcordia Technologies
   445 South Street, Room 1C258B
   Morristown, NJ 07960

   EMail: vravi@research.telcordia.com

   Adrian Smith
   BT
   BT Adastral Park
   Martlesham Heath,
   Ipswich IP5 3RE
   UK

   EMail: adrian.ca.smith@bt.com

   Vijay Srinivasan
   1200 Bridge Parkway
   Redwood City, CA 94065

   EMail: vsriniva@cosinecom.com

   Alain Vedrenne
   Equant
   Heraklion, 1041 route des Dolines, BP347
   06906 Sophia Antipolis, Cedex, France

   EMail: Alain.Vedrenne@equant.com

19. Normative References

   [BGP]             Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
                     (BGP-4)", RFC 4271, January 2006.

   [BGP-MP]          Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
                     "Multiprotocol Extensions for BGP-4", RFC 2858,
                     June 2000.

   [BGP-EXTCOMM]     Sangli, S., Tappan, D., and Y. Rekhter, "BGP
                     Extended Communities Attribute", RFC 4360, February
                     2006.

   [MPLS-ARCH]       Rosen, E., Viswanathan, A., and R. Callon,
                     "Multiprotocol Label Switching Architecture", RFC
                     3031, January 2001.

   [MPLS-BGP]        Rekhter, Y. and E. Rosen, "Carrying Label
                     Information in BGP-4", RFC 3107, May 2001.

   [MPLS-ENCAPS]     Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                     Farinacci, D., Li, T., and A. Conta, "MPLS Label
                     Stack Encoding", RFC 3032, January 2001.

20. Informative References

   [BGP-AS4]         Vohra, Q. and E. Chen, "BGP Support for Four-Octet
                     AS Number Space", Work in Progress, March 2004.

   [BGP-ORF]         Chen, E. and Y. Rekhter, "Cooperative Route
                     Filtering Capability for BGP-4", Work in Progress,
                     March 2004.

   [BGP-RFSH]        Chen, E., "Route Refresh Capability for BGP-4", RFC
                     2918, September 2000.

   [BGP-RR]          Bates, T., Chandra, R., and E. Chen, "BGP Route
                     Reflection - An Alternative to Full Mesh IBGP", RFC
                     2796, April 2000.

   [IANA]            Narten, T. and H. Alvestrand, "Guidelines for
                     Writing an IANA Considerations Section in RFCs",
                     BCP 26, RFC 2434, October 1998.

   [MPLS-ATM]        Davie, B., Lawrence, J., McCloghrie, K., Rosen, E.,
                     Swallow, G., Rekhter, Y., and P. Doolan, "MPLS
                     using LDP and ATM VC Switching", RFC 3035, January
                     2001.

   [MPLS/BGP-IPsec]  Rosen, E., De Clercq, J., Paridaens, O., T'Joens,
                     Y., and C. Sargor, "Architecture for the Use of
                     PE-PE IPsec Tunnels in BGP/MPLS IP VPNs", Work in
                     Progress, March 2004.

   [MPLS-FR]         Conta, A., Doolan, P., and A. Malis, "Use of Label
                     Switching on Frame Relay Networks Specification",
                     RFC 3034, January 2001.

   [MPLS-in-IP-GRE]  Worster, T., Rekhter, Y., and E. Rosen,
                     "Encapsulating MPLS in IP or Generic Routing
                     Encapsulation (GRE)", RFC 4023, March 2005.

   [MPLS-LDP]        Andersson, L., Doolan, P., Feldman, N., Fredette,
                     A., and B. Thomas, "LDP Specification", RFC 3036,
                     January 2001.

   [MPLS-RSVP]       Awduche, D., Berger, L., Gan, D., Li, T.,
                     Srinivasan, V., and G. Swallow, "RSVP-TE:
                     Extensions to RSVP for LSP Tunnels", RFC 3209,
                     December 2001.

   [OSPFv2]          Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
                     1998.

   [PASTE]           Li, T. and Y. Rekhter, "A Provider Architecture for
                     Differentiated Services and Traffic Engineering
                     (PASTE)", RFC 2430, October 1998.

   [RIP]             Malkin, G., "RIP Version 2", STD 56, RFC 2453,
                     November 1998.

   [OSPF-2547-DNBIT] Rosen, E., Psenak, P., and P. Pillay-Esnault,
                     "Using an LSA Options Bit to Prevent Looping in
                     BGP/MPLS IP VPNs", Work in Progress, March 2004.

   [TCP-MD5]         Heffernan, A., "Protection of BGP Sessions via the
                     TCP MD5 Signature Option", RFC 2385, August 1998.

   [VPN-MCAST]       Rosen, E., Cai, Y., and J. Wijsnands, "Multicast in
                     MPLS/BGP VPNs", Work in Progress, May 2004.

   [VPN-OSPF]        Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF
                     as the PE/CE Protocol in BGP/MPLS VPNs", Work in
                     Progress, February 2004.

Authors' Addresses

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719

   EMail: erosen@cisco.com

   Yakov Rekhter
   Juniper Networks
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089

   EMail: yakov@juniper.net

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