LDP session parameter

There are some timers associated with LDP. The LDP process consist of LDP discovery by which an LDP adjecency will be formed and LDP session (TCP). Note; if there are more than one links between 2 LSRs, there will be LDP adjacencies on all available links but only one LDP session between 2 LSRs(provided they are use per-platform label) .
In the below
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#
There are four timers associated;
1. Discovery hello interval;

Hello message will be sent every 5 seconds on the interface enabled with “mpls ip”. Destination IP would be all routers =224.0.0.2. This hello packet ensures that link/other end LSR has no problem/active.


2. Discovery hello hold interval;
After adjacency and the LDP session are UP, LSR need to send hello packets every 5 sec (default). If the other end LSR doesnt hear LDP hello for 15 secs, it assumes the other end LSR is no longer using the label space/not involved in label switching and hence bring down the adjacency and LDP session. Log similar to below will be seen;
%LDP-5-NBRCHG: LDP Neighbor 1.1.1.1:0 (1) is DOWN (Discovery Hello Hold Timer expired)
3. Session Keepalive interval;
After two LSRs formed adjacency,  they form a TCP session and to ensure the integrity of the TCP session, the LSRs send keepalive packet every 60 sec or 1/3 of negotiated hold-down timer.
4.Session Holdtime interval;
If an LSR didn’t receive the LDP keepalive/any LDP PDUs for 180 secs, the LSR will bring down the session.
%LDP-5-NBRCHG: LDP Neighbor 3.3.3.3:0 (1) is DOWN (Received error notification from peer: Holddown time expired)

More info on this in RFC. Session 2.5.5 and 2.5.6

HTH

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