Network Working Group T. Bates Request for Comments: 4456 E. Chen Obsoletes: 2796, 1966 Cisco Systems Category: Standards Track R. Chandra Sonoa Systems April 2006
BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)
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.
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Copyright (C) The Internet Society (2006).
Abstract
The Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol designed for TCP/IP internets. Typically, all BGP speakers within a single AS must be fully meshed so that any external routing information must be re-distributed to all other routers within that Autonomous System (AS). This represents a serious scaling problem that has been well documented with several alternatives proposed.
This document describes the use and design of a method known as "route reflection" to alleviate the need for "full mesh" Internal BGP (IBGP).
Typically, all BGP speakers within a single AS must be fully meshed and any external routing information must be re-distributed to all other routers within that AS. For n BGP speakers within an AS that requires to maintain n*(n-1)/2 unique Internal BGP (IBGP) sessions. This "full mesh" requirement clearly does not scale when there are a large number of IBGP speakers each exchanging a large volume of routing information, as is common in many of today's networks.
This scaling problem has been well documented, and a number of proposals have been made to alleviate this [2,3]. This document represents another alternative in alleviating the need for a "full mesh" and is known as "route reflection". This approach allows a BGP speaker (known as a "route reflector") to advertise IBGP learned routes to certain IBGP peers. It represents a change in the commonly understood concept of IBGP, and the addition of two new optional non-transitive BGP attributes to prevent loops in routing updates.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [7].
Route reflection was designed to satisfy the following criteria.
o Simplicity
Any alternative must be simple to configure and easy to understand.
o Easy Transition
It must be possible to transition from a full-mesh configuration without the need to change either topology or AS. This is an unfortunate management overhead of the technique proposed in [3].
o Compatibility
It must be possible for noncompliant IBGP peers to continue to be part of the original AS or domain without any loss of BGP routing information.
These criteria were motivated by operational experiences of a very large and topology-rich network with many external connections.
In ASX, there are three IBGP speakers (routers RTR-A, RTR-B, and RTR-C). With the existing BGP model, if RTR-A receives an external
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route and it is selected as the best path it must advertise the external route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers) will not re-advertise these IBGP learned routes to other IBGP speakers.
If this rule is relaxed and RTR-C is allowed to advertise IBGP learned routes to IBGP peers, then it could re-advertise (or reflect) the IBGP routes learned from RTR-A to RTR-B and vice versa. This would eliminate the need for the IBGP session between RTR-A and RTR-B as shown in Figure 2 below.
We use the term "route reflection" to describe the operation of a BGP speaker advertising an IBGP learned route to another IBGP peer. Such a BGP speaker is said to be a "route reflector" (RR), and such a route is said to be a reflected route.
The internal peers of an RR are divided into two groups:
1) Client peers
2) Non-Client peers
An RR reflects routes between these groups, and may reflect routes among client peers. An RR along with its client peers form a cluster. The Non-Client peer must be fully meshed but the Client peers need not be fully meshed. Figure 3 depicts a simple example outlining the basic RR components using the terminology noted above.
When an RR receives a route from an IBGP peer, it selects the best path based on its path selection rule. After the best path is selected, it must do the following depending on the type of peer it is receiving the best path from
1) A route from a Non-Client IBGP peer:
Reflect to all the Clients.
2) A route from a Client peer:
Reflect to all the Non-Client peers and also to the Client peers. (Hence the Client peers are not required to be fully meshed.)
An Autonomous System could have many RRs. An RR treats other RRs just like any other internal BGP speakers. An RR could be configured to have other RRs in a Client group or Non-client group.
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In a simple configuration, the backbone could be divided into many clusters. Each RR would be configured with other RRs as Non-Client peers (thus all the RRs will be fully meshed). The Clients will be configured to maintain IBGP session only with the RR in their cluster. Due to route reflection, all the IBGP speakers will receive reflected routing information.
It is possible in an Autonomous System to have BGP speakers that do not understand the concept of route reflectors (let us call them conventional BGP speakers). The route reflector scheme allows such conventional BGP speakers to coexist. Conventional BGP speakers could be members of either a Non-Client group or a Client group. This allows for an easy and gradual migration from the current IBGP model to the route reflection model. One could start creating clusters by configuring a single router as the designated RR and configuring other RRs and their clients as normal IBGP peers. Additional clusters can be created gradually.
Usually, a cluster of clients will have a single RR. In that case, the cluster will be identified by the BGP Identifier of the RR. However, this represents a single point of failure so to make it possible to have multiple RRs in the same cluster, all RRs in the same cluster can be configured with a 4-byte CLUSTER_ID so that an RR can discard routes from other RRs in the same cluster.
When a route is reflected, it is possible through misconfiguration to form route re-distribution loops. The route reflection method defines the following attributes to detect and avoid routing information loops:
ORIGINATOR_ID
ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type code 9. This attribute is 4 bytes long and it will be created by an RR in reflecting a route. This attribute will carry the BGP Identifier of the originator of the route in the local AS. A BGP speaker SHOULD NOT create an ORIGINATOR_ID attribute if one already exists. A router that recognizes the ORIGINATOR_ID attribute SHOULD ignore a route received with its BGP Identifier as the ORIGINATOR_ID.
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CLUSTER_LIST
CLUSTER_LIST is a new, optional, non-transitive BGP attribute of Type code 10. It is a sequence of CLUSTER_ID values representing the reflection path that the route has passed.
When an RR reflects a route, it MUST prepend the local CLUSTER_ID to the CLUSTER_LIST. If the CLUSTER_LIST is empty, it MUST create a new one. Using this attribute an RR can identify if the routing information has looped back to the same cluster due to misconfiguration. If the local CLUSTER_ID is found in the CLUSTER_LIST, the advertisement received SHOULD be ignored.
The BGP Decision Process Tie Breaking rules (Sect. 9.1.2.2, [1]) are modified as follows:
If a route carries the ORIGINATOR_ID attribute, then in Step f) the ORIGINATOR_ID SHOULD be treated as the BGP Identifier of the BGP speaker that has advertised the route.
In addition, the following rule SHOULD be inserted between Steps f) and g): a BGP Speaker SHOULD prefer a route with the shorter CLUSTER_LIST length. The CLUSTER_LIST length is zero if a route does not carry the CLUSTER_LIST attribute.
Care should be taken to make sure that none of the BGP path attributes defined above can be modified through configuration when exchanging internal routing information between RRs and Clients and Non-Clients. Their modification could potentially result in routing loops.
In addition, when a RR reflects a route, it SHOULD NOT modify the following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED. Their modification could potentially result in routing loops.
The BGP protocol provides no way for a Client to identify itself dynamically as a Client of an RR. The simplest way to achieve this is by manual configuration.
One of the key component of the route reflection approach in addressing the scaling issue is that the RR summarizes routing information and only reflects its best path.
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Both Multi-Exit Discriminators (MEDs) and Interior Gateway Protocol (IGP) metrics may impact the BGP route selection. Because MEDs are not always comparable and the IGP metric may differ for each router, with certain route reflection topologies the route reflection approach may not yield the same route selection result as that of the full IBGP mesh approach. A way to make route selection the same as it would be with the full IBGP mesh approach is to make sure that route reflectors are never forced to perform the BGP route selection based on IGP metrics that are significantly different from the IGP metrics of their clients, or based on incomparable MEDs. The former can be achieved by configuring the intra-cluster IGP metrics to be better than the inter-cluster IGP metrics, and maintaining full mesh within the cluster. The latter can be achieved by
o setting the local preference of a route at the border router to reflect the MED values, or
o making sure the AS-path lengths from different ASes are different when the AS-path length is used as a route selection criteria, or
o configuring community-based policies to influence the route selection.
One could argue though that the latter requirement is overly restrictive, and perhaps impractical in some cases. One could further argue that as long as there are no routing loops, there are no compelling reasons to force route selection with route reflectors to be the same as it would be with the full IBGP mesh approach.
To prevent routing loops and maintain consistent routing view, it is essential that the network topology be carefully considered in designing a route reflection topology. In general, the route reflection topology should be congruent with the network topology when there exist multiple paths for a prefix. One commonly used approach is the reflection based on Point of Presence (POP), in which each POP maintains its own route reflectors serving clients in the POP, and all route reflectors are fully meshed. In addition, clients of the reflectors in each POP are often fully meshed for the purpose of optimal intra-POP routing, and the intra-POP IGP metrics are configured to be better than the inter-POP IGP metrics.
The authors would like to thank Dennis Ferguson, John Scudder, Paul Traina, and Tony Li for the many discussions resulting in this work. This idea was developed from an earlier discussion between Tony Li and Dimitri Haskin.
In addition, the authors would like to acknowledge valuable review and suggestions from Yakov Rekhter on this document, and helpful comments from Tony Li, Rohit Dube, John Scudder, and Bruce Cole.
The pictorial description of the encoding of the CLUSTER_LIST attribute is removed as the description is redundant to the BGP specification, and the attribute length field is inadvertently described as one octet.
All the changes listed in Appendix A, plus the following.
Several terminologies related to route reflection are clarified, and the reference to EBGP routes/peers are removed.
The handling of a routing information loop (due to route reflection) by a receiver is clarified and made more consistent.
The addition of a CLUSTER_ID to the CLUSTER_LIST has been changed from "append" to "prepend" to reflect the deployed code.
The section on "Configuration and Deployment Considerations" has been expanded to address several operational issues.
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Authors' Addresses
Tony Bates Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134
EMail: tbates@cisco.com
Ravi Chandra Sonoa Systems, Inc. 3255-7 Scott Blvd. Santa Clara, CA 95054
EMail: rchandra@sonoasystems.com
Enke Chen Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134
EMail: enkechen@cisco.com
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