Route Reflector Split Horizon Rules

Classic IBGP: IBGP routes are not propagated to other IBGP peers.

Route reflector can propagate IBGP routes to other IBGP peers.

Classic IBGP: IBGP routes are not propagated to other IBGP peers.

Full mesh of IBGP peers is therefore required.

Route reflector can propagate IBGP routes to other IBGP peers.

Full mesh of IBGP peers is no longer required.

© 2004 Cisco Systems, Ii

In classic IBGP, the BGP boundary router needs to forward the route that is received from an EBGP peer to each and every other router within its own AS using a dedicated IBGP session for each one. Also, the BGP boundary router forwards routes that are sourced by a router in the same way. To allow every router to update every other router, a full mesh of IBGP sessions is required.

The IBGP route reflector design relaxes the need for a full mesh. The router, configured as a route reflector, under certain conditions, will relay updates that are received through an IBGP session to another IBGP session. This capability requires modifications of the classic IBGP split-horizon rules.

The route reflector concept introduces processing overhead on the concentration router and, if it is configured incorrectly, can cause routing loops and instability.

Copyright © 2004, Cisco Systems, Inc. Scaling Service Provider Networks 6-23

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Route Reflector Split-Horizon Rules (Cont.)

© 2004 Cisco Systems, Inc. All rights re

© 2004 Cisco Systems, Inc. All rights re

When you implement a route-reflector-based IBGP network, the BGP routers are divided into route reflectors (which implement modified split-horizon rules) and clients (which are behaving like traditional IBGP routers).

Route reflector clients are excluded from the full mesh. They can have any number of EBGP sessions but may have only one IBGP session, the session with their route reflector. Clients conform to the classic IBGP split-horizon rules and forward a received route from EBGP on their IBGP neighbor sessions. But the route reflector conforms to the route reflector split-horizon rules and recognizes that it has an IBGP session to a client. When the IBGP update is received from the client, the route reflector forwards the update to other IBGP neighbors, therefore alleviating the IBGP full-mesh requirement for its clients.

Similarly, when the route reflector receives an IBGP update from a neighbor that is not its client, it will forward the update to all of its clients.

Forwarding of an IBGP update in a route reflector does not change the next-hop attribute or any other common BGP attribute. This feature means that the client will use the most optimum route by means of recursive routing, regardless of the way that it has received the BGP route.

6-24 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Was this article helpful?

+1 0

Post a comment