This article helps to troubleshoot "RPD_LDP_SESSIONDOWN" or "RPD_LDP_NBRDOWN" log messages.

Customer reported LDP peers are going down and below logs are seen:

Apr 13 15:54:04 re0.router1 rpd[27365]: RPD_LDP_SESSIONDOWN: LDP session 10.10.10.38 is down, reason: hold time expired
Apr 13 15:54:20 re0.router1 rpd[27365]: RPD_LDP_NBRDOWN: LDP neighbor 10.10.10.38 (lo0.0) is down

Troubleshooting Steps:

Step 1. Check reason for LDP session going down in log messages:

Apr 13 15:54:04 re0.router1 rpd[27365]: RPD_LDP_SESSIONDOWN: LDP session 10.10.10.38 is down, reason: hold time expired

Step 2. Check configuration for LDP peer if it is direct/indirect LDP neighbor.

Step 3. If it is direct peering then check if ISIS adjacency is up or not. If ISIS is flapping need to troubleshoot the same.

Step 4. In this case, we checked that neighbor 10.10.10.38 is configured under l2circuit.

l2circuit {

 neighbor 10.10.10.38 {

      interface ae0.1217 {

        virtual-circuit-id 407116;

        control-word;

        mtu 9150;

        encapsulation-type ethernet;

      }

Step 5. Check if there are any log messages of l2circuit going down during same timestamp.

Apr 13 15:53:40 re0.router1 rpd[27365]: RPD_LAYER2_VC_DOWN: State of Layer 2 VC (Neighbor : 10.10.10.38, VC-ID : 492673) changed from UP to VC-Dn

Apr 13 15:53:40 re0.router1 rpd[27365]: RPD_LAYER2_VC_DOWN: State of Layer 2 VC (Neighbor : 10.10.10.38, VC-ID : 9767) changed from UP to VC-Dn

Step 6. Check relevant physical interface logs , check if interface flapped during same time.

Step 7. If yes, check if any errors reported on FPC and troubleshoot for fpc issues.

Step 8. In this case, physical interface or ae bundle was not flapped, only logical interface flapping observed. No other anomalies were seen on Juniper end.
Thus suggested customer to check at remote end for any flaps.

Customer checked peer device and resolved routing issue in peer device to resolve issue.

https://juniper.lightning.force.com/lightning/r/Knowledge__kav/ka03c0000009T6QAAU/view#L2VPN