BGP CLUSTER_LIST is a small but important route-reflector attribute. This note preserves the original explanation and adds a clearer operational frame for troubleshooting iBGP route reflection.
What CLUSTER_LIST does
CLUSTER_LIST is a optional nontransitive attribute.Is a list of Cluster IDs that an update has traversed. When a Route Reflector sends a route, received from a client to a non-client, it appends the local Cluster ID to the CLUSTER_LIST.
It is another loop prevention mechanism used in Route Refelction idea. When RR received an update, it first checks the CLUSTER_LIST. If it sees its own Cluster ID in the list, it knows that a routing loop has occured and ignores the update.
Related reading
- Start Here: Networking Topics
- BGP label hub
- Routing label hub
- BGP Route Reflectors: Scaling iBGP with a Simple Test Diagram
- BGP Communities: The Small Routing Tag That Powers Large Service Provider Networks
Discussion
If you have tested this in a lab or seen a different behavior in production, share the platform, software version and troubleshooting notes in the comments. Real results are more useful than generic theory.
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