Click the exhibit. When router A receives the BGP update for the 192.168.0.1/27 prefix, it propagates it to routers B and C. What action is taken by router B when it receives the update?

Question

Options

• ARouter B propagates the BGP update to routers D and C.
• BRouter B propagates the BGP update to router D.
• CRouter B detects duplicates in its BGP database and discards the update.
• DRouter B propagates the update to routers A, C and D.

• A
  3% (1)
• B
  78% (25)
• C
  6% (2)
• D
  13% (4)

Explanation

Option B is correct because of the BGP split-horizon rule: a router will not re-advertise a route learned from an iBGP (internal BGP) peer to another iBGP peer. Since A, B, and C are all in the same AS (iBGP peers), B received the update from A via iBGP - so B cannot forward it to C (who already received it from A anyway). However, D is in a different AS, making it an eBGP peer of B, so B can and does propagate the update to D.

Why the distractors are wrong:

• A is wrong because B cannot re-advertise an iBGP-learned route to C (split-horizon rule).
• C is wrong because B doesn't simply discard the update - it still forwards it to its eBGP neighbor D.
• D is wrong because B cannot send the update back to A (the source), and sending to C violates iBGP split-horizon, and this topology doesn't support all three destinations.

Memory tip: Think of iBGP as a "one-hop trust" - routes learned inside the AS from an iBGP peer stay inside and only exit to eBGP neighbors. "iBGP stops at the border; eBGP carries it across."

No community discussion yet for this question.