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CCNP SP MPLS FAQ: MPLS Operation and Maintenance

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CCNP SP MPLS FAQ: MPLS Operation and Maintenance

Q1. What does MPLS LSP ping provide that IP ping does not?

Answer: IP ping verifies only IP connectivity, whereas LSP ping also verifies whether the LSP is broken.

Q2. Name three MPLS-specific MIBs.

Answer: MPLS-specific MIBs can include any three of the following:

  • MPLS-LDP-MIB
  • MPLS-LSR-MIB
  • MPLS-TE MIB
  • MPLS-VPN-MIB
  • CISCO-IETF-PW-MIB
  • CISCO-IETF-PW-MPLS-MIB
  • CISCO-IETF-PW-TC-MIB

Q3. What does a ping mpls or traceroute mpls with reply mode router-alert do?

Answer: The return packet has the IP Router Alert option. The Router Alert label is present if the return packet is labeled.

Q4. What is the name for the Cisco IOS subsystem that is responsible for processing MPLS echo request and echo reply packets?

Answer: LSPV is the Cisco IOS subsystem that is responsible for processing MPLS echo request and echo reply packets.

Q5. What MPLS-related information can you see with MPLS-aware Netflow?

Answer: With MPLS-aware Netflow, you can see the following MPLS-related information:

  • Value of the labels
  • Experimental bits value
  • MPLS application that is associated with the top label
  • Prefix that is associated with the top label
  • Label position
  • End-of-Stack bit

Q6. What prevents an MPLS echo request packet from being forwarded as an IP packet after it becomes unlabeled?

Answer: The IP TTL is set to 1, and the destination IP address is from the range 127.0.0.0/8.

Q7. What is IP SLA?

Answer: IP SLA is an embedded tool in Cisco IOS that does network performance measurement. It can measure jitter, one-way delay, round-trip-time (RTT), and packet loss by sending probes that can be of different protocols.

Q8. If Type 1 VCCV is used, how is the packet identified as being an AToM VCCV packet?

Answer: If Type 1 VCCV is used, the control word below the label stack has the first nibble equal to 1.

Q9. Why would you want to change the default destination IP address of 127.0.0.1 in the IP header of LSP ping packets if the packets are label-switched anyway?

Answer: If you have load-balanced paths in the MPLS network, changing the destination IP address can change the path that the packet takes.

Q10. What is context-based access for SNMP?

Answer: A context can be associated with a VRF on a PE router to enable only hosts in the VPN to access the MIB data on the PE router that pertains only to that VRF.
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