A network engineer is troubleshooting an IPv6 routing issue. A router is unable to reach a specific IPv6 network despite having a valid entry in its routing table. Upon inspection, it's noted that the entry shows the route as 'installed', but the 'next-hop' address is a link-local address (e.g., FE80::1) on an interface that is currently down. Which of the following is the most likely reason for the routing failure?
- AThe next-hop link-local address is not resolvable due to the interface being down.
- BThe IPv6 network address in the routing table is incorrect.
- CThe router is missing a default IPv6 route.
- DIPv6 unicast routing is not enabled on the router.
Show answer & explanationAnswer & explanation
Correct answer: A. The next-hop link-local address is not resolvable due to the interface being down.
While a link-local address can be a valid next-hop for IPv6 routes, it is only valid for traffic exiting the specific interface on which that link-local address is configured. If that interface is down, the router cannot forward packets to that next-hop, rendering the route unusable even if it appears in the routing table as 'installed'. The route effectively becomes invalid because its exit path is unavailable.
Why the other options are wrong
- B. The problem states there is a 'valid entry' in the routing table and implies the network address itself is correct, shifting the focus to the next-hop's reachability.
- C. A missing default route would affect all unknown destinations, but the problem specifies a 'specific IPv6 network' with a valid entry.
- D. If IPv6 unicast routing (`ipv6 unicast-routing`) was not enabled, no IPv6 routes would be installed or used at all, which contradicts the 'valid entry' and 'installed' status.
IPv6 Link-Local Next-Hop
An IPv6 link-local address (FE80::/10) can be used as a next-hop in a routing table, but it is only relevant for the specific local link (interface) it is associated with.
- Only valid on the local segment/interface.
- Requires the outgoing interface to be specified (or implied).
- If the interface is down, the link-local next-hop is unreachable.
Memory trick: IPv6 routes need a clear path and a live next-hop.