Which of the following descriptions about the Cluster_List attribute in BGP is wrong?

Question

Options

• AWhen RR receives a BGP route carrying the Cluster_List attribute, and the attribute value contains the Cluster_ID of the cluster, RR considers that there is a loop in this route, so it will ignore the update relevant to this route
• BThe attribute is an optional transitive type
• CThe Cluster_List is formed by the route reflector, which consists of a series of Cluster_ID
• DWhen a route is reflected by the reflector, the Cluster_ID of the RR will be added to the Cluster_List attribute of the route

• A
  10% (3)
• B
  84% (26)
• C
  3% (1)
• D
  3% (1)

Explanation

Option B is wrong because the Cluster_List attribute is optional non-transitive, not optional transitive. Since Cluster_List is used solely within an autonomous system to prevent routing loops among route reflectors, it has no meaning outside that AS and must not be forwarded to external peers - which is exactly what the non-transitive flag enforces.

Options A, C, and D are all accurate: A correctly describes the loop-detection mechanism where an RR discards any update whose Cluster_List already contains its own Cluster_ID; C correctly defines Cluster_List as an ordered sequence of Cluster_IDs accumulated as a route passes through reflectors; and D correctly states that an RR appends its own Cluster_ID to this list before reflecting a route onward.

Memory tip: Transitive attributes travel across AS boundaries (think of BGP Community or AS_Path), but Cluster_List is purely an intra-AS housekeeping attribute - it stops at the AS edge, making it non-transitive by design.

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