Basic of LDP

Label Distribution Protocol;

  1. LSR sends LDP hello message with Destination IP= 224.0.0.2 and Src/Des UPD port = 646, once “mpls ip” command is enabled on an interface. This process is called “Discovery”
  2. Hello messages will be sent every 5 secs (By default). Default Parameters can be seen with “show mpls ldp parameters”

R0#show mpls ldp parameters

Protocol version: 1
No label generic region for downstream label distribution
Session hold time: 180 sec; keep alive interval: 60 sec
Discovery hello: holdtime: 15 sec; interval: 5 sec
Discovery targeted hello: holdtime: 90 sec; interval: 10 sec
Downstream on Demand max hop count: 255
Downstream on Demand Path Vector Limit: 255
LDP for targeted sessions
LDP initial/maximum backoff: 15/120 sec
LDP loop detection: off
R0#
  • LDP Discovery hello interval/hold time interval can be changed by
R0(config)#mpls ldp discovery hello interval 10
R0(config)#mpls ldp discovery hello holdtime 80
  • LDP packets will have below information;
  1. LSR ID: Can be set by “mpls ldp router-id loopback 0 force ” Command.
  2. Label Space: Always 0 for “Platform-wide label space”
  3. whether the hello packet is Targeted.
  4. IPv4 Transport Address. By Default, it is same as LSR ID. So, Make sure the LSR ID is reachable from the other LSR to form neighborship.  LSR with Highest IP address among the neighbor becomes “active” and initialize the LDP TCP session towards “passive” LSR.  Can be manually set by “mpls ldp discovery transport-address  x.x.x.x” interface command
If LDP session is not coming UP check the route to Neighbor transport IP;
R3#show mpls ldp discovery
Local LDP Identifier:
10.2.1.2:0
Discovery Sources:
Interfaces:
FastEthernet0/0 (ldp): xmit/recv
LDP Id: 100.100.100.100:0; IP addr: 12.12.12.12; no route
R3#
When a transport address is reachable, LDP initialization message is sent.

R3#
*Mar  1 00:34:15.943: %LDP-5-NBRCHG: LDP Neighbor 100.100.100.100:0 (1) is UP
R3#
R3#show mpls ldp neighbor
Peer LDP Ident: 100.100.100.100:0; Local LDP Ident 10.2.1.2:0
TCP connection: 12.12.12.12.39595 – 10.2.1.2.646
State: Oper; Msgs sent/rcvd: 17/19; Downstream
Up time: 00:11:40
LDP discovery sources:
FastEthernet0/0, Src IP addr: 10.2.1.1
Addresses bound to peer LDP Ident:
10.2.1.1        100.100.100.100 12.12.12.12
R3#
In case of multi-link between LSR, one LDP TCP session will be formed;
R0#show mpls ldp neighbor
Peer LDP Ident: 1.1.1.1:0; Local LDP Ident 100.100.100.100:0
TCP connection: 1.1.1.1.646 – 100.100.100.100.61483
State: Oper; Msgs sent/rcvd: 13/11; Downstream
Up time: 00:04:04
LDP discovery sources:
FastEthernet0/0, Src IP addr: 10.1.1.2
FastEthernet0/1, Src IP addr: 10.1.2.2
Addresses bound to peer LDP Ident:
10.1.1.2        10.1.2.2        1.1.1.1
R0#
HTH
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