2-5 - Troubleshoot Route Summarization Issues

Question 1

Describe route summarization.

Answer 1

• Summarization reduces the number of routing updates that flood across areas or routing domains.
• Route summarization simplifies route tables by replacing many specific addresses with a single address.

Question 2

What are typical examples of networks that requre end-to-end LSPs?

Answer 2

• An MPLS VPN backbone.
• A network that uses Cisco MPLS TE.
• An MPLS-enabled ATM network.
• A transit BGP AS, where the core routers do not run BGP.

Question 3

What transport protocol and port number does MPLS ping use?

Answer 3

UDP 3603

Question 4

Describe the two types of messages for MPLS pings.

Answer 4

• MPLS echo request: includes info about the tested FEC (prefix), which encodes as one of the TLVs; additional TLVs can be used to request more info:
  *downstream router and interface
  *MTU
  *multipath info
• MPLS echo reply: uses same format but may include additional TLVs to encode the info; basic reply is encoded in the reply code field

Question 5

T/F: By default, the MPLS echo request uses the outgoing interface IP address as the source and a loopback IP address as the destination (127.0.0.1), and a TTL of 255.

Answer 5

True

Question 6

Describe the reply modes that the initiating router in an MPLS ping can request.

Answer 6

• default reply mode: uses IPv4 and MPLS to return the reply
• router alert mode: forces every router in the return path to perform process switching of the return packet (forcing them to use the IP forwarding table)
• no reply mode: request that no reply be transmitted in response to the request

Question 7

IOS/IOS-XE and IOS-XR command:
ping mpls ipv4 [ip address] [subnet mask]

Answer 7

perform an MPLS ping to a destination address

Question 8

IOS/IOS-XE and IOS-XR command:
ping mpls ipv4 [ip address] [subnet mask] ttl 1 dsmap repeat 1

Answer 8

• perform an MPLS ping to a destination address with the dsmap option set, TTL of 1, and repeat of 1
• dsmap option retrieves details for a given hop

Question 9

IOS/IOS-XE and IOS-XR command:
traceroute mpls ipv4 [ip address] [subnet mask]

Answer 9

• perform an MPLS ping to a destination address
• you can determine labels and MTU by using MPLS traceroute. Detailed error information is retrieved upon failure somewhere in the path.

Question 10

Which of the following statements about route summarization is true?

A. Do not use route summarization where an end-to-end LSP is necessary.
B. Regular ping and traceroute commands help operators to monitor LSP issues.
C. Route summarization increases the number of routing updates that flood.
D. Use MPLS LSP ping and traceroute to identify LDP breakages.

Answer 10

A