Question 230
An iBGP PE announces external reachability to core peers. Observed: Peers see the advertised prefix but cannot install it in their RIBs. After the correction, which result would most convincingly show that this fault is resolved?
Explanation
Correct Answer
After repair IGP reachability or apply an appropriate next-hop-self policy at the ingress border, inspect whether the prefix becomes active with a resolvable BGP next hop. This closes the loop between the reported observation (Peers see the advertised prefix but cannot install it in their RIBs) and the underlying issue (The advertised BGP next hop is unreachable in the underlay).
Incorrect Answers
This result is useful in another case with evidence Traffic selects an unwanted exit despite the closer contracted carrier. It does not test recovery from peers see the advertised prefix but cannot install it in their ribs. Check instead whether the prefix becomes active with a resolvable BGP next hop.
That observation would verify a separate fault involving pe loopback reachability is missing from the transport igp. Its symptom is One site receives the route with a next hop it cannot resolve. For this case the actual issue is the advertised bgp next hop is unreachable in the underlay, and the relevant success signal is The prefix becomes active with a resolvable BGP next hop.
It confirms an unrelated service correction: The reflector has no alternate path because add-path or path-diversity strategy is absent. The present case reports Peers see the advertised prefix but cannot install it in their RIBs. Its completion test is The prefix becomes active with a resolvable BGP next hop.