Internet Engineering Task Force (IETF) A. Farrel
Request for Comments: 9125
Old Dog Consulting
Category: Standards Track J. Drake
ISSN: 2070-1721 E. Rosen
Gateway Auto-Discovery and Route Advertisement for Site Interconnection
Using Segment Routing
Data centers are attached to the Internet or a backbone network by
gateway routers. One data center typically has more than one gateway
for commercial, load-balancing, and resiliency reasons. Other sites,
such as access networks, also need to be connected across backbone
networks through gateways.
This document defines a mechanism using the BGP Tunnel Encapsulation
attribute to allow data center gateway routers to advertise routes to
the prefixes reachable in the site, including advertising them on
behalf of other gateways at the same site. This allows segment
routing to be used to identify multiple paths across the Internet or
backbone network between different gateways. The paths can be
selected for load-balancing, resilience, and quality purposes.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9125
Copyright (c) 2021 IETF Trust and the persons identified as the
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Table of Contents 1.
Requirements Language 3.
Site Gateway Auto-Discovery 4.
Relationship to BGP - Link State and Egress Peer Engineering 5.
Advertising a Site Route Externally 6.
IANA Considerations 8.
Security Considerations 9.
Manageability Considerations 9.1.
Relationship to Route Target Constraint 10.
Normative References 10.2.
Data centers (DCs) are critical components of the infrastructure used
by network operators to provide services to their customers. DCs
(sites) are interconnected by a backbone network, which consists of
any number of private networks and/or the Internet. DCs are attached
to the backbone network by routers that are gateways (GWs). One DC
typically has more than one GW for various reasons including
commercial preferences, load balancing, or resiliency against
connection or device failure.
Segment Routing (SR) ([RFC8402
]) is a protocol mechanism that can be
used within a DC as well as for steering traffic that flows between
two DC sites. In order for a source site (also known as an ingress
site) that uses SR to load-balance the flows it sends to a
destination site (also known as an egress site), it needs to know the
complete set of entry nodes (i.e., GWs) for that egress DC from the
backbone network connecting the two DCs. Note that it is assumed
that the connected set of DC sites and the border nodes in the
backbone network on the paths that connect the DC sites are part of
the same SR BGP - Link State (LS) instance (see [RFC7752
]) so that traffic engineering using SR may be used for these
Other sites, such as access networks, also need to be connected
across backbone networks through gateways. For illustrative
purposes, consider the ingress and egress sites shown in Figure 1 as
separate Autonomous Systems (ASes) (noting that the sites could be
implemented as part of the ASes to which they are attached, or as
separate ASes). The various ASes that provide connectivity between
the ingress and egress sites could each be constructed differently
and use different technologies such as IP; MPLS using global table
routing information from BGP; MPLS IP VPN; SR-MPLS IP VPN; or SRv6 IP
VPN. That is, the ingress and egress sites can be connected by
tunnels across a variety of technologies. This document describes
how SR Segment Identifiers (SIDs) are used to identify the paths
between the ingress and egress sites.
The solution described in this document is agnostic as to whether the
transit ASes do or do not have SR capabilities. The solution uses SR
to stitch together path segments between GWs and through the
Autonomous System Border Routers (ASBRs). Thus, there is a
requirement that the GWs and ASBRs are SR capable. The solution
supports the SR path being extended into the ingress and egress sites
if they are SR capable.
The solution defined in this document can be seen in the broader
context of site interconnection in [SR-INTERCONNECT]. That document
shows how other existing protocol elements may be combined with the
solution defined in this document to provide a full system, but it is
not a necessary reference for understanding this document.
Suppose that there are two gateways, GW1 and GW2 as shown in
Figure 1, for a given egress site and that they each advertise a
route to prefix X, which is located within the egress site with each
setting itself as next hop. One might think that the GWs for X could
be inferred from the routes' next-hop fields, but typically it is not
the case that both routes get distributed across the backbone: rather
only the best route, as selected by BGP, is distributed. This
precludes load-balancing flows across both GWs.
| Ingress | | Egress ------ |
| Site | | Site |Prefix| |
| | | | X | |
| | | ------ |
| -- | | --- --- |
| |GW| | | |GW1| |GW2| |
| \ | / |
| \ | / |
| -+------------- --------+--------+-- |
| ||ASBR| ----| |---- |ASBR| |ASBR| | |
| | ---- |ASBR+------+ASBR| ---- ---- | |
| | ----| |---- | |
| | | | | |
| | ----| |---- | |
| | AS1 |ASBR+------+ASBR| AS2 | |
| | ----| |---- | |
| --------------- -------------------- |
| |ASBR| |ASBR| |
| ---- AS3 ---- |
Figure 1: Example Site Interconnection
The obvious solution to this problem is to use the BGP feature that
allows the advertisement of multiple paths in BGP (known as Add-
]) to ensure that all routes to X get advertised by
BGP. However, even if this is done, the identity of the GWs will be
lost as soon as the routes get distributed through an ASBR that will
set itself to be the next hop. And if there are multiple ASes in the
backbone, not only will the next hop change several times, but the
Add-Paths technique will experience scaling issues. This all means
that the Add-Paths approach is effectively limited to sites connected
over a single AS.
This document defines a solution that overcomes this limitation and
works equally well with a backbone constructed from one or more ASes
using the Tunnel Encapsulation attribute ([RFC9012
]) as follows:
When a GW to a given site advertises a route to a prefix X within
that site, it will include a Tunnel Encapsulation attribute that
contains the union of the Tunnel Encapsulation attributes
advertised by each of the GWs to that site, including itself.
In other words, each route advertised by a GW identifies all of the
GWs to the same site (see Section 3
for a discussion of how GWs
discover each other), i.e., the Tunnel Encapsulation attribute
advertised by each GW contains multiple Tunnel TLVs, one or more from
each active GW, and each Tunnel TLV will contain a Tunnel Egress
Endpoint sub-TLV that identifies the GW for that Tunnel TLV.
Therefore, even if only one of the routes is distributed to other
ASes, it will not matter how many times the next hop changes, as the
Tunnel Encapsulation attribute will remain unchanged.
To put this in the context of Figure 1, GW1 and GW2 discover each
other as gateways for the egress site. Both GW1 and GW2 advertise
themselves as having routes to prefix X. Furthermore, GW1 includes a
Tunnel Encapsulation attribute, which is the union of its Tunnel
Encapsulation attribute and GW2's Tunnel Encapsulation attribute.
Similarly, GW2 includes a Tunnel Encapsulation attribute, which is
the union of its Tunnel Encapsulation attribute and GW1's Tunnel
Encapsulation attribute. The gateway in the ingress site can now see
all possible paths to X in the egress site regardless of which route
is propagated to it, and it can choose one or balance traffic flows
as it sees fit.
2. Requirements Language
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
3. Site Gateway Auto-Discovery
To allow a given site's GWs to auto-discover each other and to
coordinate their operations, the following procedures are
* A route target ([RFC4360
be attached to each GW's auto-
discovery route (defined below), and its value MUST
be set to a
value that indicates the site identifier. The rules for
constructing a route target are detailed in [RFC4360
]. It is RECOMMENDED
that a Type x00 or x02 route target be used.
* Site identifiers are set through configuration. The site
be the same across all GWs to the site (i.e., the
same identifier is used by all GWs to the same site) and MUST
unique across all sites that are connected (i.e., across all GWs
to all sites that are interconnected).
* Each GW MUST
construct an import filtering rule to import any
route that carries a route target with the same site identifier
that the GW itself uses. This means that only these GWs will
import those routes, and that all GWs to the same site will import
each other's routes and will learn (auto-discover) the current set
of active GWs for the site.
The auto-discovery route that each GW advertises consists of the
* IPv4 or IPv6 Network Layer Reachability Information (NLRI)
]) containing one of the GW's loopback addresses (that
is, with an AFI/SAFI pair that is one of the following: IPv4/NLRI
used for unicast forwarding (1/1); IPv6/NLRI used for unicast
forwarding (2/1); IPv4/NLRI with MPLS Labels (1/4); or IPv6/NLRI
with MPLS Labels (2/4)).
* A Tunnel Encapsulation attribute ([RFC9012
]) containing the GW's
encapsulation information encoded in one or more Tunnel TLVs.
To avoid the side effect of applying the Tunnel Encapsulation
attribute to any packet that is addressed to the GW itself, the
address advertised for auto-discovery MUST
be a different loopback
address than is advertised for packets directed to the gateway
As described in Section 1
, each GW will include a Tunnel
Encapsulation attribute with the GW encapsulation information for
each of the site's active GWs (including itself) in every route
advertised externally to that site. As the current set of active GWs
changes (due to the addition of a new GW or the failure/removal of an
existing GW), each externally advertised route will be re-advertised
with a new Tunnel Encapsulation attribute, which reflects the current
set of active GWs.
If a gateway becomes disconnected from the backbone network, or if
the site operator decides to terminate the gateway's activity, it MUST
withdraw the advertisements described above. This means that
remote gateways at other sites will stop seeing advertisements from
or about this gateway. Note that if the routing within a site is
broken (for example, such that there is a route from one GW to
another but not in the reverse direction), then it is possible that
incoming traffic will be routed to the wrong GW to reach the
destination prefix; in this degraded network situation, traffic may
Note that if a GW is (mis)configured with a different site identifier
from the other GWs to the same site, then it will not be auto-
discovered by the other GWs (and will not auto-discover the other
GWs). This would result in a GW for another site receiving only the
Tunnel Encapsulation attribute included in the BGP best route, i.e.,
the Tunnel Encapsulation attribute of the (mis)configured GW or that
of the other GWs.
4. Relationship to BGP - Link State and Egress Peer Engineering
When a remote GW receives a route to a prefix X, it uses the Tunnel
Egress Endpoint sub-TLVs in the containing Tunnel Encapsulation
attribute to identify the GWs through which X can be reached. It
uses this information to compute SR Traffic Engineering (SR TE) paths
across the backbone network looking at the information advertised to
it in SR BGP - Link State (BGP-LS) ([RFC9085
]) and correlated using
the site identity. SR Egress Peer Engineering (EPE) ([RFC9086
be used to supplement the information advertised in BGP-LS.
5. Advertising a Site Route Externally
When a packet destined for prefix X is sent on an SR TE path to a GW
for the site containing X (that is, the packet is sent in the ingress
site on an SR TE path that describes the whole path including those
parts that are within the egress site), it needs to carry the
receiving GW's SID for X such that this SID becomes the next SID that
is due to be processed before the GW completes its processing of the
packet. To achieve this, each Tunnel TLV in the Tunnel Encapsulation
attribute contains a Prefix-SID sub-TLV ([RFC9012
]) for X.
As defined in [RFC9012
], the Prefix-SID sub-TLV is only for IPv4/IPV6
Labeled Unicast routes, so the solution described in this document
only applies to routes of those types. If the use of the Prefix-SID
sub-TLV for routes of other types is defined in the future, further
documents will be needed to describe their use for site
interconnection consistent with this document.
Alternatively, if MPLS SR is in use and if the GWs for a given egress
site are configured to allow GWs at remote ingress sites to perform
SR TE through that egress site for a prefix X, then each GW to the
egress site computes an SR TE path through the egress site to X and
places each in an MPLS Label Stack sub-TLV ([RFC9012
]) in the SR
Tunnel TLV for that GW.
Please refer to Section 7
of [SR-INTERCONNECT] for worked examples of
how the SID stack is constructed in this case and how the
advertisements would work.
If a site is configured to allow remote GWs to send packets to the
site in the site's native encapsulation, then each GW to the site
will also include multiple instances of a Tunnel TLV for that native
encapsulation in externally advertised routes: one for each GW. Each
Tunnel TLV contains a Tunnel Egress Endpoint sub-TLV with the address
of the GW that the Tunnel TLV identifies. A remote GW may then
encapsulate a packet according to the rules defined via the sub-TLVs
included in each of the Tunnel TLVs.
7. IANA Considerations
IANA maintains the "BGP Tunnel Encapsulation Attribute Tunnel Types"
registry in the "Border Gateway Protocol (BGP) Tunnel Encapsulation"
IANA had previously assigned the value 17 from this subregistry for
"SR Tunnel", referencing this document as an Internet-Draft. At that
time, the assignment policy for this range of the registry was "First
Come First Served" [RFC8126
IANA has marked that assignment as deprecated. IANA may reclaim that
codepoint at such a time that the registry is depleted.
8. Security Considerations
From a protocol point of view, the mechanisms described in this
document can leverage the security mechanisms already defined for
BGP. Further discussion of security considerations for BGP may be
found in the BGP specification itself ([RFC4271
]) and in the security
analysis for BGP ([RFC4272
]). The original discussion of the use of
the TCP MD5 signature option to protect BGP sessions is found in
], while [RFC6952
] includes an analysis of BGP keying and
The mechanisms described in this document involve sharing routing or
reachability information between sites, which may mean disclosing
information that is normally contained within a site. So it needs to
be understood that normal security paradigms based on the boundaries
of sites are weakened and interception of BGP messages may result in
information being disclosed to third parties. Discussion of these
issues with respect to VPNs can be found in [RFC4364
] describes many of the issues associated with the exchange
of topology or TE information between sites.
Particular exposures resulting from this work include:
* Gateways to a site will know about all other gateways to the same
site. This feature applies within a site, so it is not a
substantial exposure, but it does mean that if the BGP exchanges
within a site can be snooped or if a gateway can be subverted,
then an attacker may learn the full set of gateways to a site.
This would facilitate more effective attacks on that site.
* The existence of multiple gateways to a site becomes more visible
across the backbone and even into remote sites. This means that
an attacker is able to prepare a more comprehensive attack than
exists when only the locally attached backbone network (e.g., the
AS that hosts the site) can see all of the gateways to a site.
For example, a Denial-of-Service attack on a single GW is
mitigated by the existence of other GWs, but if the attacker knows
about all the gateways, then the whole set can be attacked at
* A node in a site that does not have external BGP peering (i.e., is
not really a site gateway and cannot speak BGP into the backbone
network) may be able to get itself advertised as a gateway by
letting other genuine gateways discover it (by speaking BGP to
them within the site), so it may get those genuine gateways to
advertise it as a gateway into the backbone network. This would
allow the malicious node to attract traffic without having to have
secure BGP peerings with out-of-site nodes.
* An external party intercepting BGP messages anywhere between sites
may learn information about the functioning of the sites and the
locations of endpoints. While this is not necessarily a
significant security or privacy risk, it is possible that the
disclosure of this information could be used by an attacker.
* If it is possible to modify a BGP message within the backbone, it
may be possible to spoof the existence of a gateway. This could
cause traffic to be attracted to a specific node and might result
in traffic not being delivered.
All of the issues in the list above could cause disruption to site
interconnection, but they are not new protocol vulnerabilities so
much as new exposures of information that SHOULD
be protected against
using existing protocol mechanisms such as securing the TCP sessions
over which the BGP messages flow. Furthermore, it is a general
observation that if these attacks are possible, then it is highly
likely that far more significant attacks can be made on the routing
system. It should be noted that BGP peerings are not discovered but
always arise from explicit configuration.
Given that the gateways and ASBRs are connected by tunnels that may
run across parts of the network that are not trusted, data center
operators using the approach set out in this network MUST
using gateway-to-gateway encryption to protect the data center
traffic. Additionally, due consideration MUST
be given to encrypting
end-to-end traffic as it would be for any traffic that uses a public
or untrusted network for transport.
9. Manageability Considerations
The principal configuration item added by this solution is the
allocation of a site identifier. The same identifier MUST
assigned to every GW to the same site, and each site MUST
different identifier. This requires coordination, probably through a
central management agent.
It should be noted that BGP peerings are not discovered but always
arise from explicit configuration. This is no different from any
other BGP operation.
The site identifiers that are configured and carried in route targets
(see Section 3
) are an important feature to ensure that all of the
gateways to a site discover each other. Therefore, it is important
that this value is not misconfigured since that would result in the
gateways not discovering each other and not advertising each other.
9.1. Relationship to Route Target Constraint
In order to limit the VPN routing information that is maintained at a
given route reflector, [RFC4364
] suggests that route reflectors use
"Cooperative Route Filtering", which was renamed "Outbound Route
Filtering" and defined in [RFC5291
] defines an extension
to that mechanism to include support for multiple autonomous systems
and asymmetric VPN topologies such as hub-and-spoke. The mechanism
in RFC 4684
is known as Route Target Constraint (RTC).
An operator would not normally configure RTC by default for any AFI/
SAFI combination and would only enable it after careful
consideration. When using the mechanisms defined in this document,
the operator should carefully consider the effects of filtering
routes. In some cases, this may be desirable, and in others, it
could limit the effectiveness of the procedures.
10.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271
, January 2006,
] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360
, DOI 10.17487/RFC4360
February 2006, <https://www.rfc-editor.org/info/rfc4360
] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760
, January 2007,
] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925
, DOI 10.17487/RFC5925
June 2010, <https://www.rfc-editor.org/info/rfc5925
] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752
, March 2016,
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012
, April 2021,
10.2. Informative References
] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272
, DOI 10.17487/RFC4272
, January 2006,
] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364
, DOI 10.17487/RFC4364
] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684
, DOI 10.17487/RFC4684
November 2006, <https://www.rfc-editor.org/info/rfc4684
] Chen, E. and Y. Rekhter, "Outbound Route Filtering
Capability for BGP-4", RFC 5291
, DOI 10.17487/RFC5291
August 2008, <https://www.rfc-editor.org/info/rfc5291
] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952
, DOI 10.17487/RFC6952
, May 2013,
] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911
, July 2016,
] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
Ceccarelli, D., and X. Zhang, "Problem Statement and
Architecture for Information Exchange between
Interconnected Traffic-Engineered Networks", BCP 206, RFC 7926
, DOI 10.17487/RFC7926
, July 2016,
] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126
, DOI 10.17487/RFC8126
, June 2017,
] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402
, DOI 10.17487/RFC8402
July 2018, <https://www.rfc-editor.org/info/rfc8402
] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085
, August 2021,
] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086
, DOI 10.17487/RFC9086
Farrel, A. and J. Drake, "Interconnection of Segment
Routing Sites - Problem Statement and Solution Landscape",
Work in Progress, Internet-Draft, draft-farrel-spring-sr-
domain-interconnect-06, 19 May 2021,
Thanks to Bruno Rijsman, Stephane Litkowski, Boris Hassanov, Linda
Dunbar, Ravi Singh, and Daniel Migault for review comments, and to
Robert Raszuk for useful discussions. Gyan Mishra provided a helpful
GenArt review, and John Scudder and Benjamin Kaduk made helpful
comments during IESG review.
Old Dog Consulting