MPLS LDP not UP

[vin.marco]

Hello experts
I have a problem with an MPLS adjacency.
I have an ASR900 connected to a 10Gb C9300, MPLS and OSPF are enabled in this interface, adjacencies are created and everything works.
When the C9300 is restarted, the MPLS and OSPF adjacencies are not recreated, they are only created if I enter ASR900 and delete and re-enter the MPLS IP command.
Can you tell me why?

[Harold Ritter]

Hi @MHM Cisco World ,

The metric will be back to normal as soon as the LDP IGP synchronization is achieved, which should happen fairly quickly. So I don't see that as an issue.

Regards,

[MHM Cisco World]

I know it retrun but during ldp established it high and I must make @vin.marco notice this point in case he face some issue.

That all

Thanks 

MHM

[Harold Ritter]

@MHM Cisco World ,

The max-metric will be set in all cases when LDP IGP synchronization is configured. So it is not specifically relevant to the situation that the OP ran into.

Form documentation:

"If the LDP peer is reachable, the IGP waits indefinitely (by default) for synchronization to be achieved. To limit the length of time the IGP session must wait, enter the no mpls ldp igp sync holddown command. If the LDP peer is not reachable, the IGP establishes the adjacency to enable the LDP session to be established.

When an IGP adjacency is established on a link but LDP IGP synchronization is not yet achieved or is lost, the IGP advertises the max-metric on that link."

Regards,

[MHM Cisco World]

I already show this in lab I run

Thanks 

MHM

[Harold Ritter]

Hi @vin.marco ,

On startup, LDP looks at the routing table and if it sees a route to reach the LDP neighbor, it will attempts to bring up a session to that neighbor. In the meantime OSPF waits for LDP to establish the session. The issue happens when the route present in the routing table is either an aggregate or a default route and that route doesn't lead to that specific neighbor. The LDP session will obviously not come up. OSPF will wait forever by default or for the value configured with the command "mpls ldp igp sync holddown".

We can see in the output you provided that you use the default setting and OSPF will therefore wait for ever:

RMC4DFDI12002#sh mpls ldp igp sync
    TenGigabitEthernet0/1/7:
        LDP configured; LDP-IGP Synchronization enabled.

        …

        IGP holddown time: infinite.

One more thing. I see you have NSF configured on the above device. Here's a note from documentation about using NSF and the LDP IGP synchronization feature:

 "MPLS LDP IGP Synchronization Incompatibility with IGP Nonstop Forwarding

The MPLS LDP IGP Synchronization feature is not supported during the startup period if the Interior Gateway Protocol (IGP) nonstop forwarding (NSF) is configured. The MPLS LDP IGP Synchronization feature conflicts with IGP NSF when the IGP is performing NSF during startup. After the NSF startup is complete, the MPLS LDP IGP Synchronization feature is supported."

Regards,