Click the exhibit. Assume all router-IDs are properly configured, and IPv6 link-local addresses are used to establish all eBGP sessions. Given the following iBGP configuration on router A, what is…

Question

Options

• AThe IPv6 link-local address of the router D interface towards router A.
• BThe IPv6 link-local address of the router A interface towards router D.
• CThe IPv6 global address of router A.
• DThe IPv6 global address of router D.

• A
  6% (3)
• B
  4% (2)
• C
  76% (39)
• D
  14% (7)

Explanation

Option C is correct because when router A learns prefix 2001:DB8:A:401::1/128 from router D via eBGP using an IPv6 link-local next-hop, that link-local address is only valid on the local link between A and D - it cannot be used as a next-hop by router B, which is on a different link. When router A re-advertises the prefix to iBGP peers (B and C) using their global addresses, it substitutes its own global IPv6 address as the next-hop, since that is the address router B can actually reach.

Why the distractors are wrong:

• A (Router D's link-local toward A): Link-local addresses are non-routable beyond the local link, so router B cannot use router D's link-local address as a next-hop.
• B (Router A's link-local toward D): Also a link-local - same problem. Router B is not on that link and cannot reach it.
• D (Router D's global address): Standard iBGP does not preserve the eBGP peer's global address as next-hop in this scenario; router A replaces it with its own global when advertising to iBGP peers.

Memory tip: Think "link-local dies at the link." Whenever an eBGP session runs on link-local addresses and the route crosses into iBGP, the advertising router must swap in its own routable (global) address as the next-hop - link-locals simply cannot travel beyond their local segment.

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