The network is exhibiting behavior, including packet loss, which corresponds to an MTU conflict with one or more devices on the network path.

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

MTU, or Maximum Transmission Unit, is also known as "link MTU". It represents the maximum packet size that can be sent in one piece over a link (or hop) without being fragmented. It is measured in bytes. It differs from the "path MTU" (PMTU) which is the minimum MTU of the hops in a path.

In order to avoid this type of conflict, the "DF" (Don't Fragment) bit in the IP header of a packet can be used to dynamically discover the PMTU of a path. The basic idea is that a source host initially assumes that the PMTU of a path is the (known) MTU of its first hop, and sends all packets on that path with the DF bit set. If any of the packets are too large to be forwarded without fragmentation by some router along the path, that router will discard them and return ICMP "destination unreachable" messages with a code meaning "fragmentation required but DF set".

Upon receipt of such a message, also known as a "packet too big" message, the source host reduces its assumed PMTU for the path. The PMTU discovery process ends when the host's estimate of the PMTU is low enough that its packets can be delivered without fragmentation or packet loss. Or, the host may elect to end the discovery process by ceasing to set the DF bit in the packet headers. It may do so, for example, because it is willing to have packets fragmented in some circumstances. Normally, the host continues to set DF in all packets, so that if the route changes and the new PMTU is lower, it will be discovered.

Possible secondary messages

  • "Unusual condition - should be detected as a gray or black hole" Path MTU discovery normally identifies this condition as a gray-hole hop or a black-hole hop, so this message indicates that something unusual has happened with MTU and that it should be inspected carefully.
  • "Large packet size has been set higher than the measured MTU of the end-to-end path" The advanced test setting "Large Packet Size" has been set to a value that is too high.
  • "Set the specified value for MTU lower than the measured MTU or apply Use Measured MTU setting" Set the advanced test setting "Large Packet Size" to a lower value, or set it to "Use Measured MTU", before rerunning your test.

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