Network Working Group T. Bates (Cisco Systems)
Internet Draft R. Chandra (Sonoa Systems)
Expiration Date: April 2006 E. Chen (Cisco Systems)
BGP Route Reflection -
An Alternative to Full Mesh IBGP
draft-ietf-idr-rfc2796bis-02.txt
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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 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 the need for "full mesh" IBGP.
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This documents obsoletes RFC 2796 and RFC 1966.
1. Introduction
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 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 "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.
This documents obsoletes RFC 2796 [6] and RFC 1966 [4].
2. Specification of Requirements
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].
3. Design Criteria
Route Reflection was designed to satisfy the following criteria.
o Simplicity
Any alternative must be both simple to configure as well as
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].
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o Compatibility
It must be possible for non compliant IBGP peers to continue 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.
4. Route Reflection
The basic idea of Route Reflection is very simple. Let us consider
the simple example depicted in Figure 1 below.
+-------+ +-------+
| | IBGP | |
| RTR-A |--------| RTR-B |
| | | |
+-------+ +-------+
\ /
IBGP \ ASX / IBGP
\ /
+-------+
| |
| RTR-C |
| |
+-------+
Figure 1: Full Mesh IBGP
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 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.
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+-------+ +-------+
| | | |
| RTR-A | | RTR-B |
| | | |
+-------+ +-------+
\ /
IBGP \ ASX / IBGP
\ /
+-------+
| |
| RTR-C |
| |
+-------+
Figure 2: Route Reflection IBGP
The Route Reflection scheme is based upon this basic principle.
5. Terminology and Concepts
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 a RR are divided into two groups:
1) Client Peers
2) Non-Client Peers
A RR reflects routes between these groups, and may reflect routes
among client peers. A 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.
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/ - - - - - - - - - - - - - -
| Cluster |
+-------+ +-------+
| | | | | |
| RTR-A | | RTR-B |
| |Client | |Client | |
+-------+ +-------+
| \ / |
IBGP \ / IBGP
| \ / |
+-------+
| | | |
| RTR-C |
| | RR | |
+-------+
| / \ |
- - - - - /- - -\- - - - - - /
IBGP / \ IBGP
+-------+ +-------+
| RTR-D | IBGP | RTR-E |
| Non- |---------| Non- |
|Client | |Client |
+-------+ +-------+
Figure 3: RR Components
6. Operation
When a 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 the 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. A RR treats other RRs just
like any other internal BGP speakers. A 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 a 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 co-exist. Conventional BGP speakers
could be either members of 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.
7. Redundant RRs
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.
8. Avoiding Routing Information Loops
When a route is reflected, it is possible through mis-configuration
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 a 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 which recognizes the ORIGINATOR_ID attribute SHOULD ignore a
route received with its BGP Identifier as the ORIGINATOR_ID.
CLUSTER_LIST
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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 a 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 mis-
configuration. If the local CLUSTER_ID is found in the CLUSTER_LIST,
the advertisement received SHOULD be ignored.
9. Impact on Route Selection
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.
10. Implementation Considerations
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 potential result in routing loops.
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11. Configuration and Deployment Considerations
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.
Both MEDs and 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 which 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.
o or by making sure the AS-path lengths from different ASs are
different when the AS-path length is used as a route selection
criteria.
o or by configuring community based policies using which the
reflector can decide on the best route.
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 congruent with the network topology when
there exist multiple paths for a prefix. One commonly used approach
is the POP-based reflection, 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,
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and the intra-POP IGP metrics are configured to be better than the
inter-POP IGP metrics.
12. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing IBGP [1, 5].
13. Acknowledgments
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.
14. References
14.1. Normative References
[1] Rekhter, Y., T. Li and S. Hares, "A Border Gateway Protocol 4
(BGP-4)", draft-ietf-idr-bgp4-26.txt, October 2004.
14.2. Informative References
[2] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh
routing", RFC 1863, October 1995.
[3] Traina, P., "Limited Autonomous System Confederations for BGP",
RFC 1965, June 1996.
[4] Bates, T. and R. Chandra, "BGP Route Reflection An alternative
to full mesh IBGP", RFC 1966, June 1996.
[5] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[6] Bates, T., R. Chandra and E. Chen "BGP Route Reflection - An
Alternative to Full Mesh IBGP", RFC 2796, Arpil 2000.
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[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
15. 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
16. Appendix A Comparison with RFC 2796
The impact on route selection is added.
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.
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17. Appendix B Comparison with RFC 1966
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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19. Full Copyright Notice
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
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Bates, Chandra, Chen [Page 12]
