BGP Route Reflection An alternative to full mesh IBGP
A technique to reduce the complexity of internal BGP network topology.
Bates, T. and R. Chandra, "BGP Route Reflection An alternative to full mesh IBGP", RFC 1966, DOI 10.17487/RFC1966, June 1996, <https://www.rfc-editor.org/info/rfc1966>.
@misc{rfc1966,
series = {Request for Comments},
number = 1966,
howpublished = {RFC 1966},
publisher = {RFC Editor},
doi = {10.17487/RFC1966},
url = {https://www.rfc-editor.org/info/rfc1966},
author = {T. Bates and R. Chandra},
title = {{BGP Route Reflection An alternative to full mesh IBGP}},
pagetotal = 7,
year = 1996,
month = jun,
} RFC text
Contents
Network Working Group T. Bates
Request for Comments: 1966 cisco Systems
Category: Experimental R. Chandra
cisco Systems
June 1996
BGP Route Reflection
An alternative to full mesh IBGP
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
The Border Gateway Protocol [1] is an inter-autonomous system routing
protocol designed for TCP/IP internets. BGP deployments are
configured such that that 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 [2,3].
This document describes the use and design of a method known as
"Route Reflection" to alleviate the the need for "full mesh" IBGP.
1. Introduction
Currently in the Internet, BGP deployments are configured such that
that 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. This "full mesh" requirement clearly does not
scale when there are a large number of IBGP speakers as is common in
many of todays internet networks.
For n BGP speakers within an AS you must maintain n*(n-1)/2 unique
IBGP sessions. With finite resources in both bandwidth and router CPU
this clearly does not scale.
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". It represents a change in
the commonly understood concept of IBGP and the addition of two new
optional transitive BGP attributes.
2. 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 Migration
It must be possible to migrate 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 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.
3. 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 reflect IBGP learned
routes, 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.
+------ + +-------+
| | | |
| RTR-A | | RTR-B |
| | | |
+-------+ +-------+
\ /
IBGP \ ASX / IBGP
\ /
+-------+
| |
| RTR-C |
| |
+-------+
Figure 2: Route Reflection IBGP
The Route Reflection scheme is based upon this basic principle.
4. Terminology and Concepts
We use the term "Route Reflector" (RR) to represent an IBGP speaker
that participates in the reflection. 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. 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. The Client peers
should not peer with internal speakers outside of their cluster.
Figure 3 depicts a simple example outlining the basic RR components
using the terminology noted above.
/ - - - - - - - - - - - - - -\
| 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
5. Operation
When a route is received by a RR, 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 peer
Reflect to all other Clients.
2) A Route from a Client peer
Reflect to all the Non-Client peers and also to the
Client peers other than the originator. (Hence the
Client peers are not required to be fully meshed).
3) Route from an EBGP peer
Send to all the Client and Non-Client Peers.
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.
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 normal 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 ould
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.
6. Redundant RRs
Usually a cluster of clients will have a single RR. In that case, the
cluster will be identified by the ROUTER_ID 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 must be
configured with a 4-byte CLUSTER_ID so that an RR can discern routes
from other RRs in the same cluster.
7. Avoiding Routing Information Loops
As IBGP learned routes are 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. This attribute will carry the ROUTER_ID 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 route reflector
must never send routing information back to the router specified in
ORIGINATOR_ID.
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. It is encoded as follows:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| Length | value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where Length is the number of octets.
When a RR reflects a route from its Clients to a Non-Client peer, it
must append 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 is looped back to the
same cluster due to mis-configuration. If the local CLUSTER_ID is
found in the cluster-list, the advertisement will be ignored.
8. Implementation and Configuration 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. This could result is looping of routes.
In some implementations, modification of the BGP path attribute,
NEXT_HOP is possible. For example, there could be a need for a RR to
modify NEXT_HOP for EBGP learned routes sent to its internal peers.
However, it must not be possible for an RR to set on reflected IBGP
routes as this breaks the basic principle of Route Reflection and
will result in potential black holeing of traffic.
An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED,
DPA)that could change consistent route selection. This could result
in potential loops.
The BGP protocol provides no way for a Client to identify itself
dynamically as a Client to an RR configured BGP speaker and the
simplest way to achieve this is by manual configuration.
9. Security Considerations
Security issues are not discussed in this memo.
10. Acknowledgments
The authors would like to thank Dennis Ferguson, Enke Chen, 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.
11. References
[1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[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.
12. Authors' Addresses
Tony Bates
cisco Systems
170 West Tasman Drive
San Jose, CA 95134
Phone: +1 408 527 2470
EMail: tbates@cisco.com
Ravishanker Chandrasekeran
(Ravi Chandra)
cisco Systems
170 West Tasman Drive
San Jose, CA 95134
EMail: rchandra@cisco.com
AI ELI5
On this page
§1The Full Mesh Problem
When multiple BGP (Border Gateway Protocol) routers operate inside a single network, the protocol requires every internal router to exchange routing information with every other internal router. For n routers, this demands n times (n minus 1) divided by 2 unique connections.
A network with just 10 internal BGP routers needs 45 such connections. With 100 routers, it jumps to 4,950. This full mesh becomes a scaling problem: bandwidth usage grows quadratically, router CPU burden increases, and configuration becomes unwieldy.
If every kid in a class has to tell every other kid their favorite color, and there are 30 kids, that's a lot of talking. It's much easier to have one kid listen and then announce it to everyone else.
§2Route Reflection
Route Reflection relaxes the requirement that all internal BGP speakers must be fully meshed. A designated route reflector listens to external routing information and internal announcements, then selectively re-advertises (or reflects) them to other internal routers. This breaks the strict peering rule that internal peers do not re-advertise to other internal peers.
The route reflector router acts as an intermediary, allowing client routers to avoid direct peering with each other. A route reflector along with its client routers form a cluster. Client routers within a cluster need not be fully meshed; they only peer with their route reflector.
A special helper router listens to routing announcements from other routers and passes them along to the right neighbors, like a post office sorting mail.
| Aspect | Full Mesh IBGP | Route Reflection |
|---|---|---|
| Number of sessions | n(n-1)/2 | Reduced to cluster size |
| Client mesh requirement | All fully meshed | Only within cluster |
| Configuration complexity | High | Medium |
| Scalability | Poor | Good |
§3Architecture and Operation
A route reflector categorizes its internal peers into two groups: client peers and non-client peers. When the route reflector receives a route, it applies reflection rules. Routes from a non-client peer go to all client peers. Routes from a client peer go to all non-client peers and to other client peers (except the originator). Routes from external BGP peers go to both groups.
A network may have many route reflectors. Multiple route reflectors can be configured to form a backbone where they are all non-clients of each other (fully meshed among themselves) while managing clusters of client routers. This design allows gradual migration: old BGP speakers that do not understand route reflection can coexist as either client or non-client routers.
The helper has two types of helpers to manage: regular helpers who talk to each other, and special student helpers who only talk to the main helper.
- Announces route
- Reflects route
- No direct peering needed
§4Preventing Routing Loops
When routes are reflected, they can loop back to routers that already know them, especially if misconfiguration occurs. Route Reflection defines two new optional BGP attributes to detect and prevent loops.
The ORIGINATOR_ID attribute (type code 9) carries the router ID of the router that originated a route in the local network. A route reflector must never send a route back to the router specified in the ORIGINATOR_ID. The CLUSTER_LIST attribute (type code 10) is a sequence of cluster IDs representing the reflection path a route has taken. If a route reflector sees its own cluster ID in the CLUSTER_LIST, it knows the route has already passed through its cluster and discards it.
The mail gets a stamp each time a helper passes it along. If the helper sees their own stamp, they know the mail already came through them and they throw it away to prevent it going in circles.
§5Redundant Route Reflectors
A single route reflector per cluster creates a single point of failure. To provide redundancy, multiple route reflectors can serve the same cluster. When multiple route reflectors exist in a cluster, they must all be configured with a common CLUSTER_ID (a 4-byte value) so that each route reflector can identify routes coming from other reflectors in the same cluster and avoid excessive reflection.
A cluster without a configured CLUSTER_ID uses the router ID of the route reflector as its identifier. With multiple reflectors and a shared CLUSTER_ID, both reflectors can reflect routes without creating loops, and if one reflector fails, the cluster continues to work.
Instead of one helper, you can have two helpers with the same name tag, so if one is sick the other one can take over.