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    Enabling TCP PMTU Discovery

    You can enable PMTU discovery on the active virtual router using the tcp path-mtu-discovery command.

    You can use the age-timer keyword to set the time (minutes) that TCP waits before attempting to increase the path MTU after receiving an ICMP Too Big message or after previously increasing the PMTU successfully (minutes2). The range of these two timers is 1–30 minutes. The timer defaults to 10 minutes.

    You can use the age-timer indefinite keyword with the tcp path-mtu-discovery command to disable PMTU aging functions.

    To enable and configure PMTU discovery on the virtual router:

    • Issue the tcp path-mtu-discovery command in Global Configuration mode.
      host1:VR1(config)#tcp path-mtu-discovery

    To configure PMTU age timers:

    • Set path MTU discovery age timers differently.
      host1:VR1(config)#tcp path-mtu-discovery age-timer 20 15
    • Set path MTU discovery age timers to the same value (5 minutes).
      host1:VR1(config)#tcp path-mtu-discovery age-timer 5
    • Disable path MTU discovery age timers.
      host1:VR1(config)#tcp path-mtu-discovery age-timer indefinite

    Use the no version with a keyword to return the values to their defaults. Use the no version without any keywords to disable path MTU discovery on the virtual router.

    Published: 2014-08-13