Considering that both routers C and D are advertising the eBGP learned prefix 10.5.0.0/16 into ISP X, which of the following best describes the route advertisement within ISP X?

Question

Options

• ARouter C sends the update to routers E and F only.
• BRouter C sends the update to routers D, E and F.
• CRouter C sends the update to routers A, B, E and F.
• DRouter C sends the update to routers A, B, D, E and F.

• A
  4% (1)
• B
  12% (3)
• C
  4% (1)
• D
  81% (21)

Explanation

Option D is correct because when a router learns a prefix via eBGP, it advertises that prefix to all of its iBGP peers within the AS - including Router D, even though D is also advertising the same prefix externally. This is a fundamental iBGP behavior: the iBGP split-horizon rule (don't re-advertise iBGP-learned routes to other iBGP peers) does not apply here, because C's source is eBGP, not iBGP.

• A (E and F only) is wrong because it arbitrarily excludes A, B, and D with no valid BGP rule to justify it.
• B (D, E, F) is wrong because it excludes A and B - all iBGP peers receive eBGP-learned routes, regardless of their position in the topology.
• C (A, B, E, F) is the most tempting distractor: it correctly identifies most peers but omits D, perhaps under the assumption that C wouldn't notify D since D is advertising the same prefix. However, BGP does not suppress advertisements to a peer just because that peer is also originating the same prefix.

Memory tip: Remember the rule as "eBGP-in, iBGP-everywhere." Once a prefix enters the AS via eBGP, it floods to every iBGP peer in a full mesh - the split-horizon restriction only kicks in when a router tries to re-advertise something it already learned from iBGP.

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