Unexpected IGP–L2TP Subscriber Termination During Unified ISSU

Even though the Layer 2 Tunneling Protocol (L2TP) supports unified in-service software upgrade (ISSU), some of the L2TP subscribers reachable through the interior gateway protocol (IGP) (such as OSPF or IS-IS) go down during unified ISSU. Before you perform unified ISSU, set the graceful timer values of the IGP greater than the interface controller (IC) upgrade time to avoid termination of L2TP subscribers. Once unified ISSU is complete, you can configure the graceful restart timer value for the IGP as per your requirement.

Keep in mind that the IC upgrade time depends on the following factors:

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