MPLS LDP Directly Connected LDP Sessions


Most LDP sessions are established with directly connected session using directly connected interfaces.


  • Each LDP router will send out a LDP Hello message to the multicast address using port numbers UDP/646 for source and destination. The source IP will be local interface's IP.
  • Address represents All Routers on each subnet.
  • Any interface that has mpls ip enabled will multicast the LDP Hello message.
  • Upon receiving the LDP Hello message, LSR establish a TCP session using Router ID exchanged in Hello Message.
  • LDP will use the router ID as the source of TCP session. These IP has to be /32 and be in reachable.
  • To modify the router ID use the command mpls ldp router-id (inteface) [force].
  • By default router ID is selected with the highest IP of a loopback, if non exist the highest IP of any active interfaces.

Parameter Negotiations

  • Upon establishing the TCP session using TCP/646 and IP addresses of Router IDs, LSR will attempt to negotiate LDP parameters.
  • Some of the parameters include:
    • Hello and Hold timer values. If there is a mismatch with Hold Timer, the lowest value is picked. If there is a Hello timer mismatch it will be changes to match the hold timer. For example if LSR1 has hello = 1, Hold = 15 while LSR 2 has Hello = 5, Hold = 15 then hello values will not be changed. If Hold timer is changed on LSR1 to 5 sec, LSR2 will adapt the hello timer of 1 sec.
    • Label Distribution Method.
    • VPI for ATM
    • DLCI for LC-Frame Relay

Relevant Commands

mpls ldp router-id

R1(config)#        mpls ldp router-id (interface) [force]

Show mpls interface

show mpls interface

Show ldp discovery details

show  ldp discovery details

Show mpls ldp neigbhor

show mpls ldp neighbor

Show mpls ldp parameters

show mpls ldp parameters


rating: 0+x
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License