Imagine I have 4 routers connected in a full-mesh topology, 1 Autonomous System:
RouterA+----+RouterB+--+
+ +
+-RouterC--RouterD+-+
I am running BGP and I want to take advantage of router reflectors. Do I have to configure at least 2 router reflectors for the configuration above ? I don't get it. There is an exercise in a book saying that 1 router reflector would not be sufficient for the configuratio above. I understand that 2 router reflectors would give me redundancy, but I don't understand why the I can't make all BGP speakers talk using just 1 router reflector ?
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Using a Cluster List
Usually a cluster has a single route reflector, in which case, the cluster is identified by the router ID of the route reflector. To increase redundancy and avoid single points of failure, a cluster might have more than one route reflector. When a cluster has more than one route reflector, all of the route reflectors in the cluster need to be configured with a 4-byte cluster ID. The cluster ID allows route reflectors to recognize updates from other route reflectors in the same cluster.
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#wp7158
A cluster list is a sequence of cluster IDs that an update has traversed. When a route reflector sends a route from its clients to nonclients outside of the cluster, it appends the local cluster ID to the cluster list. If the route reflector receives an update whose cluster list contains the local cluster ID, the update is ignored.
In , Routers D, E, F, and H belong to the same cluster; Routers D and H are route reflectors for the same cluster. Note that Routers D and H maintain a fully meshed peering relationship with the other route reflectors in AS 100 (that is, with Routers C and G). If Router D goes down, Router H is prepared to take its place.
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