Feb 7, 2011

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 “all 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

Rob Edwards

Northern (UK) chap focusing on platforms, automation, cloud and cloud native applications. Recovering network engineer, although it turns out networking is as important, if not more, now than before!