LDP esstablishes a session in 2 steps;

  • hello messages peridically sent on all MPLS-enabled interfaces
  • MPLS enabled routers respond to received hello messages by attampting to esstablish a session with teh source of the hello message

LDP link hello is UDP sent to β€œall routers on this subnet” multicast address (

TCP is used in esstablishing the session

Both TCP and UDP use well-known LDP port number 646

A 6-byte LDP identifier (TLV) identifies the oruter  – 1st 4 bytes and label space last 2 bytes

LDP keepalive set by default every 60 sec

mpls ldp neighbor #! defines the neighbor to use, can be used for non-adjacent routers. Once neighbor discovered same mechanism to establish session is used

Applications of targetted LDP sessions;

  • MPLS Fast ReRoute (FRR)
  • MPLS NonStopForwarding (NSF)
  • MPLS LDP session protection