MPLS LDP Directly Connected LDP Sessions
Table of Contents
Introduction
Most LDP sessions are established with directly connected session using directly connected interfaces.
Discovery
- Each LDP router will send out a LDP Hello message to the multicast address 224.0.0.2 using port numbers UDP/646 for source and destination. The source IP will be local interface's IP.
- Address 224.0.0.2 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