The following are the chief characteristics of route reflectors:
• A route reflector is a router that forwards updates to its clients. When a client sends an update to the route reflector, it is forwarded or reflected to the other clients.
• The route reflector is the only router that is configured or that has the remotest idea that it is anything other than a peer.
• A client is a router that receives updates from a route reflector.
• Both a route reflector and a client, therefore, form a unit that shares information. This unit is called a cluster.
• The autonomous system is divided into clusters, and the router reflector is identified and configured. There must be at least one route reflector per cluster.
• The route reflector and client require a full peer relationship because the route reflector forwards updates from other clients, but peering between the clients is not needed.
• In all probability, a route reflector is connected to peers for which it is not forwarding routes. From the route reflector's view, these neighbors or peers are nonclients. If an update from a client is received by the route reflector, the update is forwarded to other clients as well as nonclients (both IBGP-4 and EBGP-4 peers). The only router that does have the update forwarded to it is the originator of the route.
• Nonclients must be fully meshed with the route reflector.
• The route reflector connects to other route reflectors. These route reflectors need to be fully meshed. This is to ensure that the IBGP-4 routing tables are complete.
• When the route reflector forwards an update, the Originator-ID attribute is set. This is the BGP-4 router ID of the router that originated the path. The purpose of this attribute is not to award honors to the originating router, but so that if this router receives back the update, it will see its own ID and will ignore the packet. This prevents the possibility of routing loops.
• If there are multiple route reflectors in the cluster, to provide redundancy, then the originating router is identified by the Cluster-ID attribute. This serves the same purpose as the Originator-ID in preventing routing loops.
Figure 9-5 illustrates the relationship between route reflectors, clients, and other clusters.
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