What is the function of the Cluster_List attribute?

Question

Options

• AIt is used for loop detection within the same cluster.
• BIt is used for loop detection between clusters.
• CIt is used for loop detection in a confederation.
• DIt is used to identify the route reflectors in within an AS.

• A
  3% (1)
• B
  89% (34)
• C
  3% (1)
• D
  5% (2)

Explanation

Cluster_List is a BGP path attribute used by Route Reflectors (RRs) to detect loops between clusters - each time a route is reflected out of a cluster, the RR prepends its Cluster_ID to the list, and if an RR sees its own Cluster_ID already in the list, it discards the route.

• A is wrong - Within-cluster loop prevention is handled by the ORIGINATOR_ID attribute, which tracks the originating iBGP speaker inside the cluster.
• C is wrong - Confederations use AS_CONFED_SEQUENCE / AS_CONFED_SET attributes for loop detection, not Cluster_List.
• D is wrong - Cluster_List contains Cluster_IDs (not RR identities), and its function is loop prevention, not advertisement of which routers are reflectors.

Memory tip: Think of Cluster_List as the AS_PATH equivalent for Route Reflector clusters - just as AS_PATH stops routing loops between autonomous systems by recording each AS a route passes through, Cluster_List stops loops between clusters by recording each Cluster_ID the route has crossed.

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