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    Configuring Black Hole Thresholds for TCP PMTU

    Some domains might be configured not to generate certain ICMP messages (like an ICMP destination unreachable message) or to filter all ICMP messages. Under these conditions, the source of oversized ICMP packets never learns that it is sending oversized packets. The device continues sending oversized packets that never get through. This behavior is often referred to as a black hole.

    A black hole threshold is a limit to the number of times a virtual router can retransmit identical sequences of datagrams before the retransmissions are identified as a problem.

    To specify the number of permitted retransmissions before the retransmissions are determined to be a problem:

    • Issue the tcp path-mtu-discovery command with the black-hole-detect-threshold keyword in Global Configuration mode.
      host1:VR1(config)#tcp path-mtu-discovery black-hole-detect-threshold 200

      Use the no version to disable black hole threshold detection.

    Published: 2014-08-13