Question 590
An L3VPN reuses the same OSPF domain ID across overlapping services. Observed: Sham-link connectivity is missing and a customer backdoor link becomes preferred. After the correction, which result would most convincingly show that this fault is resolved?
Explanation
Correct Answer
After implement a supported OSPF sham link where the service design requires equal intra-area semantics, inspect whether the customer selects the intended VPN path under controlled backdoor tests. This closes the loop between the reported observation (Sham-link connectivity is missing and a customer backdoor link becomes preferred) and the underlying issue (The provider path appears less preferable to the direct intra-area backdoor path).
Incorrect Answers
This result is useful in another case with evidence The PE does not export the agreed customer prefixes into its VRF. It does not test recovery from sham-link connectivity is missing and a customer backdoor link becomes preferred. Check instead whether the customer selects the intended VPN path under controlled backdoor tests.
That observation would verify a separate fault involving mp-bgp control-plane reachability is out of sync with mpls transport forwarding. Its symptom is VPN route remains active at PEs despite loss of the underlying labeled transport. For this case the actual issue is the provider path appears less preferable to the direct intra-area backdoor path, and the relevant success signal is The customer selects the intended VPN path under controlled backdoor tests.
It confirms an unrelated service correction: Export and import route-target changes were not staged compatibly. The present case reports Sham-link connectivity is missing and a customer backdoor link becomes preferred. Its completion test is The customer selects the intended VPN path under controlled backdoor tests.