Path MTU discovery

This test determines the path MTU (Maximum Transmission Unit) between two Test Agents. It allows you to find out whether the MTU values configured in network elements are appropriate. This in turn is important in order to avoid packet fragmentation.

Test procedure and fail criteria

The algorithm sends UDP frames of increasing size, starting at the Minimum MTU that you specify as the smallest acceptable value, and stopping at the smaller of the MTUs configured on the server and client Test Agent interfaces (as described here).

The test fails if the path MTU is found to be smaller than Minimum MTU. If this happens, you need to increase the MTU setting in network elements in the path, if possible. If not, the MTU setting must be lowered on the Test Agent interfaces instead.



  • Server: The server Test Agent interface.
  • Client: The client Test Agent interface.
  • Direction: The direction of the test traffic: from client to server, or vice versa.
  • Minimum MTU: The smallest acceptable MTU value. This cannot be set higher than the smaller of the MTUs configured on the Test Agent interfaces, or the test will end with an error. Default: 1500 bytes.
  • Server port: The UDP server port to use on the client. Default: 7000.
  • Wait for ready: Time to wait before starting the test. The purpose of inserting a wait is to allow all Test Agents time to come online and acquire good time sync. Min: 1 min. Max: 24 hours. Default: "Don't wait", i.e. zero wait time.
Result metrics
  • Calculated path MTU with pass/fail indication
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