CCIE SP MPLS FAQ: MPLS Troubleshooting
Q1. What are the prerequisites for successful MPLS deployment?
Answer: MPLS requires CEF enabled globally and on all interfaces on which the label imposition needs to be performed.
Q2. What are the reasons that a TDP neighbor would not be discovered?
Answer: A TDP neighbor would not be discovered because of a protocol mismatch (TDP versus LDP) or an access-list blocking TDP or LDP hello packets.
Q3. Which command can you use to display TDP neighbors?
Answer: You can use the show tag-switching tdp discovery command for a brief overview or show tag-switching tdp neighbor command for in-depth details of each TDP session.
Q4. What are the reasons that a TDP session might not start?
Answer: The TDP session will not start if the LSR has no route to the TDP identifier of adjacent LSR or if an access list is blocking the TCP session between the LSRs.
Q5. In some cases, the LSR runs TDP with the neighbors but does not assign any labels locally. What is the reason for this behavior?
Answer: CEF switching is disabled in the LSR. Local labels are assigned only to prefixes in the FIB, which is built by CEF switching mechanism.
Q6. Why would an LSR assign a label but not propagate it to its peers?
Answer: This symptom would most likely occur because of a misconfigured label distribution access list.
Q7. Why would an LSR label IP packets that it receives through one interface but not through another?
Answer: Inbound CEF switching must be operational on interfaces receiving IP packets that need label imposition. IP packets received through interfaces that do not operate in CEF switching mode are forwarded as IP packets and not labeled.
Q8. How would you discover a broken LSP in your network?
Answer: The best way to discover a broken LSP is to perform a trace with TTL propagation disabled.
Q9. Why would the introduction of MPLS break the propagation of large IP datagrams? How would you discover this symptom?
Answer: Introduction of MPLS might break the propagation of large IP datagrams because of an additional header (label header) being inserted between the Layer-2 header and Layer-3 payload. The easiest way to detect this symptom is to use the extended ping command with varying packet sizes.