MPLS LDP-IGP synchronization

[Volodymyr Khrystenko]

Hi.

I have some misunderstanding with LDP-OSPF synchrinization. As I understood, when synchronization is turned on on interface, OSPF process will not establish adjacency until LDP session comes or synchronization timer expires. But if timer expires OSPF comes up anyway and advertises LSAs with the worst metric set to 65536. If after that LDP comes up, OSPF advertises LSAs with normal metric. Am I right? And what if LDP session is not established above OSPF adjacency? What then?

Thanks.

[Edison Ortiz · Accepted solution]

It will maintain the worst metric indefinitely by default. You can implement the command

mpls ldp igp sync holddown if you wish to forward unlabeled flows in the event the LDP session never comes up.

[Harold Ritter]

Hi Edison,

The max metric will remain for ever even if you configure the hold down timer to be very short. The only way for traffic to be forwarded down this path is if this path is the only path to destination.

Regards

[Edison Ortiz]

Stand corrected, thanks Harold

[Harold Ritter · Accepted solution]

Hi Volodymyr,

The only reason for the IGP to wait for teh LDP session to come up is if there is a route to the LDP neighbor transport address that has been discovered by the LDP hello. Otherwise, the IGP will proceed with the neighbor relationship establishment, which will allow LDP to come up. During the period of time LDP is still down, the ospf metric for the specific link is set to max metric (65535). Once LDP is up, synchronization has been achieved and the metric is change from max metric to normal.

Regards