Additional Route Reflector Loop Prevention Mechanisms

• Every time a route is reflected, the router-ID of the originating IBGP router is stored in the originator-ID BGP attribute.

• A router receiving an IBGP route with originator-ID set to its own router-ID ignores that route.

• BGP path selection procedure is modified to take into account cluster-list and originator-ID.

When a route is reflected, the route reflector will set the originator-ID BGP attribute (nontransitive optional BGP attribute) to the router-ID of the peer from which it received the route. Any router that receives a route with its own router-ID in the originator-ID attribute silently ignores that route.

BGP path selection rules have been modified to select the best route in scenarios where a router might receive reflected and nonreflected routes or several reflected routes:

■ The traditional BGP path selection parameters (weight, local preference, origin, MED) are compared first.

■ If these parameters are equal, the routes that are received from EBGP neighbors are preferred over routes that are received from IBGP neighbors.

■ When a router receives two IBGP routes, the nonreflected routes (routes with no originator-ID attribute) are preferred over reflected routes.

■ The reflected routes with shorter cluster-lists are preferred over routes with longer cluster-lists.

■ If the additional route-reflector-oriented selection criteria do not yield a decision, the rest of the traditional BGP path selection rules are followed.

Copyright © 2004, Cisco Systems, Inc. Scaling Service Provider Networks 6-31

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