Question 170
An IPv6 IS-IS deployment has working IPv4 reachability. Observed: Routers do not install IPv6 interrouter loopback prefixes. After the correction, which result would most convincingly show that this fault is resolved?
Explanation
Correct Answer
After enable supported IS-IS IPv6 reachability on required interfaces and levels, inspect whether the target IPv6 loopbacks install with correct IS-IS next hops. This closes the loop between the reported observation (Routers do not install IPv6 interrouter loopback prefixes) and the underlying issue (IPv6 topology or address-family advertisements are not enabled as intended).
Incorrect Answers
This result is useful in another case with evidence Customer service interface accidentally forms an IS-IS adjacency. It does not test recovery from routers do not install ipv6 interrouter loopback prefixes. Check instead whether the target IPv6 loopbacks install with correct IS-IS next hops.
That observation would verify a separate fault involving is-is hello packet padding exceeds one path's available frame size. Its symptom is Neighbor remains in initialization and three-way handshake does not complete. For this case the actual issue is ipv6 topology or address-family advertisements are not enabled as intended, and the relevant success signal is The target IPv6 loopbacks install with correct IS-IS next hops.
It confirms an unrelated service correction: Wide metrics differ or the installed multipath limit excludes an otherwise equal path. The present case reports Routers do not install IPv6 interrouter loopback prefixes. Its completion test is The target IPv6 loopbacks install with correct IS-IS next hops.