Route Reflector Clusters

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• A group of redundant route reflectors and their clients form a cluster.

• Each cluster must have a unique cluster-ID.

• Each time a route is reflected, the cluster-ID is added to the cluster-list BGP attribute.

• The route that already contains the local cluster-ID in the cluster-list is not reflected.

© 2004 Cisco Systems, Inc. All rights reserved. BGP v3.1—6-9

A router that is acting as a route reflector client does not require any specific configuration. It simply has fewer IBGP sessions than it would have if it were part of the full mesh. But improperly configuring the client to also be a reflector could easily cause a loop. An IBGP route coming in from one of the real reflectors to the client could be forwarded by the client, erroneously acting as reflector, to the other reflector.

Route reflector clusters prevent IBGP routing loops in redundant route reflector designs.

The role of the network designer is to properly identify which route reflectors and their clients will form a cluster. The designer assigns to the cluster a cluster-ID number that is unique within the AS.

Note The cluster-ID number must be configured in the route reflectors. The clients should not be configured with this information.

A route reflector router can reflect routes only within a single cluster. A route reflector can, however, participate in another cluster but only as a client. A client can function as a client only to a route reflector belonging to the same cluster.

6-28 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.

When a route is reflected, the reflector creates the cluster-list attribute and attaches it to the route if it does not already exist. It then sets its cluster-ID number in the cluster-list or adds its cluster-ID number to an already existing cluster-list attribute. If the route, for any reason, is ever reflected back to the same reflector, it will recognize its cluster-ID number in the cluster-list and not forward it again. The first route reflector that reflects the route also sets an additional BGP attribute, called "originator-ID," and adds it to the BGP router-ID of its client.

Note The cluster-list and originator-ID attributes are nontransitive optional BGP attributes, allowing routers that do not support route reflector functionality to coexist with route reflectors and their clients in the same AS.

Based on cluster-list and originator-ID attributes, routers can implement two loop prevention mechanisms:

■ Any router that receives an IBGP update with the originator-ID attribute set to its own BGP router-ID will ignore that update.

■ Any route reflector that receives an IBGP update with its cluster-ID already in the cluster-list will ignore that update.

Copyright © 2004, Cisco Systems, Inc. Scaling Service Provider Networks 6-29

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 Clusters (Cont.)

© 2004 Cisco Systems, Inc. All rights re

© 2004 Cisco Systems, Inc. All rights re

In a cluster with redundant route reflectors, the client will forward the received EBGP update to both reflectors. The route reflectors forward the update into the IBGP full mesh. This behavior means that they will send the update to each other as well. But when a route reflector receives a BGP update from another route reflector, it will recognize their common cluster-ID number in the cluster-list attribute. Therefore, the newly received route update is ignored.

6-30 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.

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