Click the exhibit. Over which BGP session will router B learn the 10.1.1.0/24 prefix?

Question

Options

• AThe session between router A and router B.
• BThe session between router C and router B.
• CThe session between router D and router B.
• DRouter B will learn the prefix over all of the sessions, then it will choose the best route.

• A
  76% (51)
• B
  15% (10)
• C
  6% (4)
• D
  3% (2)

Explanation

Option A is correct because of the iBGP split-horizon rule: a route learned from an iBGP peer cannot be re-advertised to another iBGP peer. Router A is the source (or the eBGP peer injecting) the 10.1.1.0/24 prefix, so it advertises it directly to Router B over their iBGP session.

Why the distractors are wrong:

• B and C are wrong because Routers C and D learned the 10.1.1.0/24 prefix from an iBGP peer (Router A); the split-horizon rule prevents them from re-advertising that prefix to Router B, another iBGP peer.
• D is wrong because BGP does not flood prefixes across all sessions - the split-horizon rule specifically blocks C and D from forwarding the route, so Router B only receives it from A.

Memory tip: Think of iBGP as a "dead end" - a prefix learned over iBGP stops there and won't be passed to other iBGP neighbors. The only exceptions are route reflectors or confederations, which are explicitly designed to work around this rule. If you see a full-mesh iBGP scenario with no RR/confederation, the prefix travels only one iBGP hop from its source.

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