LDP and TDP

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When I started writing this blog post in Word, I thought I would throw all the MPLS acronyms into one blog post. I have realized it is way too long and would probably make your guys fall asleep, so I have decided to break it down by acronym. We are first going to look at LDP and TDP

MPLS introduces a few new terms you will see mentioned differently but have the same meaning.  For instance, the term “tag” can be interchanged with the term “label,” and you will see some of the output from Cisco IOS referring to some as “tag” and others as “label.”

The first acronyms we are going to explore are TDP and LDP.

  • TDP: Tag Distribution Protocol – Older technology and Cisco proprietary
  • LDP: Label Distribution Protocol – Newer, open standard, and is the default on all Cisco MPLS configurations

Forming a TDP/LDP Neighborship;

  • Hello messages are sent out to every MPLS-enabled interface
  • If a neighbor is seen out of an interface configured for MPLS, the neighbor tries to establish a connection with his MPLS neighbor using his source address for the hello messages.
  • If a neighborship is agreed upon, the LDP session is established from the router with the higher IP address
Example of an LDP neighborship


LDP/TDP Hello Messages:

  • UDP is used for hello messages that are sent to multicast IP 224.0.0.2 (All routers on this subnet multicast address)
  • TCP is used to establish the neighborship
  • TDP/LDP both use well-known TCP/UDP port numbers
    • LDP
      – 646
    • TDP
      – 711

A few things to note;

  • You must have layer 3 connectivity to your neighbor for the LDP/TDP session to form.
  • You cannot mix and match LDP and TDP in a neighborship; however, you can have TDP and LDP mixed inside our MPLS domain.
  • If you have two equal-cost links to the same LDP neighbor and MPLS is running on both, only one link will form a neighborship.
  • If your neighborhood does not come up and you can verify layer 3 connectivity, you can look at one of the following ;
    • You can check to see if an ACL is blocking TDP or LDP ports
    • Check to make sure MPLS is enabled on the interface