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
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.|
|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.
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"