Common Issues with Label Distribution

By Anthony Sequeira on December 4th, 2012

Enjoy this excerpt from the MPLS Operation and Troubleshooting text from IPexpert.

Label Distribution has a number of issues that can surface when deployed. The most common problems relate to the exchange of the essential control and forwarding plane information. The control plane establishment in LDP is very streamlined. For simplicity in troubleshooting common issues encountered during Label Distribution, we identify four categories of problems: LDP session startup Issues, No Label Allocation, or Allocated labels are not distributed between peers.

LDP Session Startup Issues

There are a relatively small number of issues that can result in a failure to see session establishment take place between LDP peers. These issues can most notably be isolated if we find the answers to the following questions:

  • Do all expected neighbors appear?
    • show mpls ldp discovery
  • Is MPLS on all needed interfaces?
    • show mpls interface
  • Do we have all expected direct adjacencies?
    • traceroute (look for more than 1 hop)
  • Are all neighbors using the same protocol (LDP/TDP)?
    • show mpls interface detail
  • Are there any ACLs or Filters blocking ports 711 or 646?
    • show ip int | inc line|list
  • Lastly, verify reachability between loopbacks
    • ping

No Label Allocation

We have spent exhaustive amount of time up to this point to learn what processes LDP is responsible for. We know that LDP relies on the underlying IGP protocol to provide a list of prefixes that will have labels bound to them. Before we can hope advertise or exchange these labels we must know that they are being created in the first place. Issues evolving a failure to create a label locally can most notably be isolated by if we find the answers to the following questions:

  • Are labels being assigned to local prefixes?
    • show mpls forwarding-table
  • Ensure CEF is enabled globally or on interfaces?
  • show cef interface

Allocated Labels Are Not Distributed

Once we have determined that LDP creating labels as we would expect it to, the next operational role for the protocol is to communicate these label to ip bindings to all adjacent or non-adjacent LDP peers. For the purposes of the Quickfire Troubleshooting process there are only a handful of issues that can prevent the successful exchange of this information, and these causes can best be isolated by finding the answers to following questions:

  • Does adjacent LSRs display received labels?
    • show mpls ldp bindings
  • Is conditional label advertising configured?
  • show run | inc advert

Anthony Sequeira
Twitter: @compsolv
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    Tags: CCIE, MPLS

    One Response to “Common Issues with Label Distribution”

    1. Jason says:

      A very good book I must say!

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