Customer observed BGP down & outage to connected sites.

Issue was resolved with changing isis key to plaintext.

As per the logs, we can see the issue was triggered with LDP session down to 4 peers.

This seems to have led to reachability to lo0 of router & inturn BGP down.

Customer was going with traffic migration from MX to ACX nodes & issue happened in ACX (with IP swap pending).

On further checks, they identified that they were using one key on MX box, with lsp-lifetime as 65535 & a different key on ACX.

The isis adjacency was Up , as the interface / hello-authentication-key was matching and the lsp-lifetime 65535 sec (18.2 hrs) configured. 

After expiration of lsp-lifetime of MX, the route disappear from isis-database, resulting in BGP down on ACX.