Click the exhibit. Assuming a full iBGP mesh in ISP 1, how does router R1 in ISP 1 handle a BGP update received from ISP 2?

Question

Options

• AThe update is sent to router R2 in ISP 1.
• BThe update is sent to routers R2 and R3 in ISP 1.
• CThe update is sent to all routers in ISP 1.
• DThe update is sent to routers R2 and R3 in ISP 1, and to router R3 in ISP 2.

• A
  3% (1)
• B
  7% (2)
• C
  90% (27)

Explanation

When R1 receives a BGP update via eBGP from ISP 2, it can freely re-advertise that update to all of its iBGP peers within ISP 1 - and in a full iBGP mesh, R1 has a direct iBGP session with every other router in ISP 1, so every router receives the update. This is the key distinction: the iBGP split-horizon rule (routes learned from an iBGP peer cannot be re-advertised to other iBGP peers) does not apply here because the source is an eBGP peer.

• A is wrong because limiting the update to only R2 is incomplete - a full mesh means R1 peers with all routers, not just one.
• B is wrong for the same reason; it arbitrarily stops at R2 and R3, implying there are only two other routers, but "all routers in ISP 1" is the correct scope regardless of count.
• D is wrong because R1 does not send iBGP updates back to routers in ISP 2 - ISP 2 is a separate AS, and iBGP sessions stay within a single AS.

Memory tip: Think "eBGP in → iBGP flood." When a route arrives from outside the AS (eBGP), it gets distributed to all internal (iBGP) peers. The split-horizon rule only blocks iBGP-to-iBGP re-advertisement, never eBGP-to-iBGP.

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