Route Reflectors

The benefits of route reflectors include the following:

• The capability to scale the network, given the other characteristics

• A strong hierarchical design

• A reduction of traffic on the network

• A reduction in the memory and CPU to maintain TCP sessions

• Faster convergence because synchronization is not needed

• Faster convergence and a simpler network because two routing protocols are implemented:

— IBGP-4 for external routing information traversing the autonomous system

— IGP for routes internal to the autonomous system Characteristics of route reflectors are as follows:

• 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 is configured. There must be at least one route reflector per cluster; otherwise, the clients will not get the updates reflected to them.

• The route reflector and the client no longer require a full peer relationship because the route reflector forwards updates from other clients.

• In all probability, a route reflector is connected to peers for whom it is not forwarding routes. These are regular neighbors or peers but, from the route reflector's view, they are nonclients.

• 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 because the old rule of not propagating routes that are not defined in the network command is now operational. 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 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.

The rules by which route reflectors propagate updates are as follows:

• If a route reflector receives multiple paths to the same destination, it chooses the best path.

• If the route is received from a client, then the route reflector reflects/forwards the update to clients and nonclients, except for the originator of the route.

• If the route is received from a nonclient, then the route reflector reflects the update only to clients.

• If the route is received from EBGP-4, then the route reflector reflects it to all nonclients as well as clients.

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