LDP (Label Distribution Protocol)

LDP is the protocol that distributes label bindings for FECs between LSRs (Label Switching Routers.) The LIB (Label Information Base) of an MPLS enabled router is the table that all the bindings are kept. The LIB is populated by information from LDP, MP-BGP and RSVP.
LDP carries the labels for interior routes, from the IGP.
MP-BGP distributes routes for BGP prefixes, and RSVP is used in MPLS-TE.

LDP Functions:
Discovery of other LSRs
Session establishment, management
Advertisement of Labels
Housekeeping, notification

Two types of Adjacency:

Hello adjacency: LDP Hellos to 224.0.0.2 (all routers multicast), using UDP Port 646 for both source and destination. These hellos are sent on all MPLS enabled interfaces. Hello/Holdtime = 5/15sec default.




Transport session: After the Hello discovery, the LSRs will establish a TCP transport session on TCP/646. One of them will take active role (initiating the TCP connection to port 646, the other will take the passive role, listening on TCP/646) The LSR with the higher transport IP address will take the passive role.

LSRs will exchange session parameters with Initialization Messages, which contain info as LDP version, label distribution method, timer values, etc.

If they agree a Transport Session is established. If not, it is re-tried. This cycle has an LDP initial/maximum backup timers with default 15/120 seconds.

The session is kept open as long as an LDP message or a Keepalive is heard. The session holdtime is 180 seconds. The interval of Keepalive messages is 60 sec.

You can list the ldp parameters used by the router:


Example keep alive messages. Sent back and forth in 60sec intervals.

Useful commands
mpls ldp router-id interface [force]
mpls ldp discovery transport-address {interface | ip-address}
mpls ldp discovery {hello {holdtime | interval} seconds
mpls ldp backoff initial-backoff maximum-backoff
mpls ldp holdtime seconds

show mpls ldp discovery detail
show mpls ldp neighbor neighbor-ip detail
show mpls ldp parameters

Ref: LDP neighbor discovery.. @Networkers-Online