Excessive ICMP "fragmentation needed and DF set" messages were received. MTU of the path changed during test. This could be detrimental to the performance of the network.

Recommended action

Check routing protocols and attempt to establish consistent MTUs across all common paths.

Detailed explanation

This refers to a technique for using the "DF" (Don't Fragment) bit in the IP header to dynamically discover the MTU of a path. The basic idea is that a source host initially assumes that the path MTU (PMTU) 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 needed and DF set".

Upon receipt of such a 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.

Possible secondary messages

  • "Check for potential problems with MTU"
  • "Route may be changing during test"

Other resources