A device in the path is sending the wrong MTU size in the "fragmentation needed and DF set" ICMP message.

Recommended action

  • Use consistent MTU values on all devices within any given subnet
  • Use routers, not Layer 2 switches or bridges to connect devices of different MTUs.
  • Upgrade VPN devices that introduce black holes
  • Avoid reducing end-station MTUs to compensate for misconfigured networks.

Detailed explanation

A gray-hole hop is a device that returns the wrong MTU value in the "fragmentation needed and DF set" ICMP message. In many cases, this condition would cause the link to behave similarly to a link that has a black hole. Treat this condition as you would treat black-hole hops.

Possible secondary messages

  • "Replies with incorrect MTU size when packet size larger than measured path MTU"
  • "Packet loss conditions may have generated an incorrect diagnosis"
  • "Local interface reports incorrect MTU" This is an unusual gray-hole condition caused by the monitoring point’s local interface where the operating system reports the local interface to be a certain MTU value, yet Delivery monitoring measures its actual MTU to be another value.

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