Click the exhibit. Routers RR1 and RR2 are route reflectors with the same Cluster-ID. How does router RA treat the updates for prefix 192.168.1.0/27 received from routers RR1 and RR2?

Question

Options

• AA. Router RA prefers the update received from router RR1.
• BB. Router RA prefers the update received from router RR2.
• CC. Router RA flags both updates as invalid.
• DD. Router RA accepts both updates and flags them as used.

• A
  11% (5)
• B
  7% (3)
• C
  59% (26)
• D
  23% (10)

Explanation

When RR1 and RR2 share the same Cluster-ID, every route they reflect carries that Cluster-ID in the BGP CLUSTER_LIST attribute. Router RA - also configured with that same Cluster-ID - receives both updates, finds its own Cluster-ID embedded in each CLUSTER_LIST, and applies the BGP loop-prevention rule from RFC 4456: any update whose CLUSTER_LIST contains the local Cluster-ID must be discarded. Because both updates trigger this rule, RA flags both as invalid, making C correct.

Why A and B are wrong: For RA to prefer either update, at least one would have to pass the loop-prevention check. Since both carry the same Cluster-ID, neither survives - so a preference decision never happens.

Why D is wrong: RA does not accept either update. The CLUSTER_LIST check fires before path selection, so "accepts both" is factually incorrect regardless of the "flags" language.

Memory tip: Think "same Cluster-ID = I've already seen this, discard it." In a redundant RR pair, two reflectors sharing a Cluster-ID is intentional - but if a downstream router is also in that cluster, it will reject reflected routes as loops. The phrase to remember is: "My Cluster-ID in the list? Both go in the bin."

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