MPLS LDP Session and Discovery

[tsepo135maroibaki]

I understand that session hold time is for an established session, and the discovery hello intervals are for neighbor discoveries, but i do not understand why there is discovery hold time, while the new neighbors are being discovered in 5 sec interval does it mean there is a 15 sec pause when there is no new neighbors discovered on the link ?

[liviu.gheorghe · Accepted solution]

Hello @tsepo135maroibaki ,

hellos are used for two purposes - neighbor discovery and neighbor being alive. Hellos are sent at a frequency given by the hello interval - 5 seconds. If you don't receive a hello from a established neighbor within the holdtime - 15 seconds - you will declare that neighbor down.

[tsepo135maroibaki]

Thank you @liviu.gheorghe, like with an IGP (OSPF), the neighbor relationship does not necessarily mean the session is established, it means the routers are LDP capable and know each other without changing any label information, then after session establishment session timers will be used to maintain the session running parallel with the discovery timers ?.

[MHM Cisco World · Accepted solution]

the reason of there are two hold time one for hello and other for session is the LDP use UDP AND TCP 
UDP for hello
TCP for exchange the label 

MHM