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Check MTU accross a link





Method to perform a check on a link to see what the best MTU (Maximum Transmission Unit) is.





Application: command line, bash

Question/Problem Description:

When utilizing VPN or MPLS connections between networks the MTU can be difficult to ascertain. A lot of "TCP segment of a reassembled PDU" messages may appear in the TCP dump.

Slow network speed is the usual indicator.


Steps to Reproduce:  
Error Message: Slow network speed and sometimes "TCP segment of a reassembled PDU" in a TCPdump.
Defect Number:  
Enhancement Number:  
Cause: The MTU on the LoadMaster interface is statically set to 1500. Decreasing this value may be required to improve performance.

Open a command prompt or bash terminal on the remote computer.

Ping the Virtual service address using the following switches.

ping <the LM VS address> -f -l 1392 -n 100

If the command prompt or bash terminal is returning the message "Packet needs to be fragmented but DF set." then the MTU is still too large.

  • The -f switch is do not fragment.
  • The -l switch is buffer length.
  • The -n switch is the number of times it will attempt the ping in this case 100.

Increasing or decreasing the number denoted by the -l switch will increase or decrease the buffer size.

Adjust as necessary in 8bit increments.

i.e. 1400 or 1384 would be plus or minus the 8 bits from the 1392 specified in the above example.

To change the MTU on the LoadMaster navigate to:

System Configuration --> Interfaces --> Select the interface (i.e eth0, eth1, eth2 etc.) --> Link Status --> MTU




In other cases, enabling "TCPSack" (TCP Selective acknowledgement) can help with network speed issues too.

It can be enabled under "System Configuration -> logging Options -> System logfiles -> Debug Options"


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