352-001 Question #662: Real Exam Question with Answer & Explanation

The correct answer is A: Packet goes from router B to A and C.. With eBGP peerings only between B-A and C-A, Router A replaces the BGP next-hop with its own address when re-advertising routes, so traffic from Host E transits Router A before reaching Router C and Host F.

Question

Refer to the exhibit. Company A migrated from Frame Relay WAN to Metro-Ethernet E-LAN service. Router B and C have only eBGP neighbor adjacency with router A using their Metro- Ethernet IP addresses. What happens when host E sends a packet to host F?

Exhibit

Options

APacket goes from router B to A and C.
BPacket is dropped by router B.
CPacket goes from router B to C.
DPacket goes from router B to Aand is dropped by router A due to the split-horizon rule.

Explanation

With eBGP peerings only between B-A and C-A, Router A replaces the BGP next-hop with its own address when re-advertising routes, so traffic from Host E transits Router A before reaching Router C and Host F.

Common mistakes.

B. Router B has a valid BGP route to Host F's network learned from Router A via eBGP, so it has a reachable next-hop and will forward the packet rather than drop it.
C. Router B's BGP next-hop for Host F points to Router A, not Router C, because Router A replaced the next-hop with its own address during eBGP re-advertisement; direct B-to-C forwarding would only occur if next-hop-unchanged or next-hop-self pointing to C were explicitly configured.
D. BGP split-horizon is an iBGP restriction that prevents a route learned from one iBGP peer from being re-advertised to another iBGP peer; it does not apply to eBGP sessions, so Router A can freely re-advertise between Router B and Router C.

Concept tested. eBGP next-hop update behavior in shared Ethernet topology

Reference. https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

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