A Border Gateway Protocol (BGP) cluster ID identifies a group of route reflectors. Internal BGP (iBGP) routes are not advertised to iBGP peers. In order to avoid having to create a fullmesh configuration, you can configure one or more route reflectors to pass iBGP routes between iBGP routers. A route reflector and its peers form a cluster, and the route reflector is configured with a 4byte cluster ID. To increase redundancy, a cluster can have multiple route reflectors. Each route reflector in the cluster should be fully meshed and configured with the same cluster ID so that the route reflector can recognize routing updates from other route reflectors in the cluster. To configure a route reflector with a cluster ID, you should issue the bgp cluster-id cluster-id command from BGP router configuration mode.
A BGP cluster ID does not identify a single route reflector in a cluster. Each route reflector is identified by its router ID. When a cluster has only a single route reflector, the cluster ID is often configured with the route reflector's router ID. When a cluster has multiple route reflectors, the cluster ID must be the same on all of the route reflectors.
A BGP cluster ID does not identify the clusters that a route has passed through; this is the function of a cluster list. When a route reflector sends a route to or receives a route from a nonclient peer router, the route reflector appends its cluster ID to the cluster list. If no cluster list exists, a cluster list is created with the cluster ID of the route reflector. If a route reflector receives a routing update with its cluster ID in the cluster list, the routing update is ignored.
A BGP cluster ID does not identify a group of peers with the same update policies; this is the function of a peer group. Peer groups can simplify administration by enabling an administrator to simultaneously configure a group of peers with the same update policies, such as route maps, filter lists, and distribute lists. Any configuration options that are configured with the specified peer group name will be applied to members of the peer group. To define a peer group, you should issue the neighbor peer-group-name-peer-group command.
A BGP cluster ID does not identify the originator of a route; this is the function of the originator ID. A route reflector that originates a route in a local autonomous system (AS) will insert its router ID as the originator ID. If a route reflector receives a routing update with its router ID as the originator ID, the routing update is ignored.