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CCNP SP MPLS FAQ: Troubleshooting MPLS Networks

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CCNP SP MPLS FAQ: Troubleshooting MPLS Networks

Q1. What would you configure on the PE routers if you wanted to be able to see the LSRs in the output of the traceroute command from the PE routers, but not the customers performing the traceroute from the CE routers?

Answer: In that case, you would configure no mpls ip propagate-ttl forwarded.

Q2. When is the MPLS TTL not copied to the IP TTL when disposing of the labels? Why?

Answer: When MPLS TTL > IP TTL, the MPLS TTL value is not copied into the IP header. This rule prevents packets from looping forever when the command no mpls ip propagatettl (forwarded) is configured on the ingress PE routers/LSRs.

Q3. What kind of packets are sent when you traceroute in Cisco IOS?

Answer: The packets that are sent when tracerouting in Cisco IOS are UDP packets with a high destination UDP port, with increasing TTL values, starting at 1.

Q4. Why does it not make much sense to configure no mpls ip propagate-ttl on P routers?

Answer: no mpls ip propagate-ttl hides the topology of the MPLS core for the customers who are connecting to the MPLS VPN network. This command takes effect only when the IP packets are imposed with labels. This imposition occurs only for the customers’ packets at the PE routers.

Q5. How can you verify which outgoing interface is taken when load balancing labeled packets?

Answer: To verify which outgoing interface is taken when load balancing labeled packets, use the command show mpls forwarding-table labels label exact-path.

Q6. What information about the labels can MPLS-aware Netflow provide?

Answer: MPLS-aware Netflow can provide the following information:

  • Label position
  • Label value
  • Experimental bits value
  • End-of-Stack bit
  • Label type (label-associated protocol)
  • The prefix that the label is bound to

Q7. Which access lists can you associate with the command debug mpls packets?

Answer: You can associate access lists from the range 2700 to 2799.

Q8. 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.

Q9. 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.

Q10. 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.

Q11. 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.

Q12. 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.

Q13. 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.

Q14. 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.

Q15. 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.

Q16. 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.

Q17. 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.

Q18. 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.

Q19. 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.

Q20. 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.

Q21. 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.

Q22. 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.

Q23. 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 interfacesreceiving 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.

Q24. 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.

Q25. 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.