Unexpected L2TP Failover of Established Tunnels During Unified ISSU

L2TP never declares itself as unified ISSU unsafe. However, unified ISSU forces an L2TP failover for all established tunnels. Successful recovery of a tunnel and its sessions following the unified in-service software upgrade requires a successful L2TP failover resynchronization, either by the L2TP silent failover method or the L2TP failover protocol.

When the L2TP silent failover method is configured on ERX1440 router, use the l2tp retransmission command to set the retransmission retry count to 8 for the remote peers. A value of more than 7 helps ensure that the remote peers keep retransmitting control messages for the duration of the unified ISSU warm restart and the tunnels are not disconnected.

See Specifying the Number of Retransmission Attempts.

When the unified ISSU operation attempts to verify the upgrade prerequisites, a warning message is generated if any tunnels are present for which failover resynchronization is disabled.

You can use the show l2tp tunnel failover-resync disable command to identify the tunnels referred to by the warning message. The command enables filtering based upon the effective failover resynchronization mechanism:

host1#show l2tp tunnel failover-resync disable L2TP tunnel 2/1 is Up with 1 active session1 L2TP tunnel found

If a successful failover resynchronization cannot be performed for a tunnel following the upgrade, then the tunnel and all of its sessions are subject to disconnection.

L2TP automatically detects a peer L2TP disconnect after the unified in-service software upgrade is completed by detecting a control channel failure.

When peer LNSs are not configured with PPP keepalives or inactivity timeouts, you must configure an inactivity timeout for L2TP on the LAC. This timeout enables the router to detect a PPP disconnect when signaling has been dropped during the unified ISSU forwarding interruption. In the absence of this configuration, the connection at the LAC and LNS is left as logged in for an extended period of time following the upgrade.

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