From CCIE Study Wiki

Using BGP routers as Route Reflectors removes the need for a BGP network to have a full mesh of iBGP peers.

Theory

Route reflectors allow you to have a partial mesh of iBGP peers. Some routers are configured as RR servers; these servers are allowed to learn iBGP routes from their clients and then advertise them on to other iBGP peers. Routers not configured as RRs behave as normal iBGP peers, they are not even aware of the RR.

Route advertisement with the route reflector's neighbors depend on the location from which a prefix is learned:

• Learned from a client - Route is advertised to both clients and non-clients
• Route learned from non-client - Route advertised to clients, but not non-clients
• Route learned from eBGP - Advertised to both

Typically, RRs are fully meshed with other RRs, as well as with any non-RR client routers.

• A route reflector and it's clients are referred to as a cluster.

RRs use several tools to prevent loops:

• CLUSTER _LIST - RRs add their cluster ID into an attribute called CLUSTER_LIST before sending an update. When receiving an update, RRs discard prefixes in which their cluster id appears. Acts like an AS_PATH for confederations.
• ORIGINATOR_ID - This PA lists the RID of the first iBGP peer to advertise this route into the AS, if a router sees its own BGP ID in a received route, it does not use or propagate the route
• Only advertising the best routes - RR will reflect a route only if it is considered "best" in its own BGP table, limiting the routes reflected by the RR.

Commands

• neighbor {ip-address | peer-group-name} route-reflector-client (router) - configures the local router as a route reflector and the specified BGP neighbor as a route reflector client.
• bgp cluster-id id (router) - If the cluster has more then one route reflector, a cluster ID must be configured on the route reflectors so they can recognize routes that have been sent from each other.

Default Settings

• The RR cluster id by default is the BGP RID, but can be manually set

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• Route reflection can resolve an endless BGP convergence situation.

• BGP Route Reflection Troubleshooting from Bryan at IP Expert.

Online Resources

• Cisco IOS IP routing protocols configuration guide on configuring BGP route reflectors.

• Cisco Learning Module presentation on introducing route reflectors (must be registered to view).

• Overview of Route Reflectors.

• Networkers Online has an excellent article and video on route reflector basics.

• CCIE to Be shows what happens in a BGP cluster when the cluster ID is changed and how set clauses are ignored when configuring route reflectors with outbound route maps. He also shows us how route reflectors prevent routing loops.

• The CT3 Wiki has excellent articles on route reflectors and route reflector update groups.

• Notes on BGP route reflectors from CCIE Talk.

• Bradly from Community String has some simple notes on route reflectors.

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