Check MTU accross a link

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

Product:

Version:

Platform: 

Application: command line, bash

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.

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"