Important Note: Cisco routers do not have the capability to change MTU (Maximum Transmit Unit) size directly. Instead you must change the MSS (Maximum Segment Size) which is always 40 bytes smaller than MTU. For example if you require an MTU of 1400 bytes you would adjust the MSS to 1360 bytes.

Cisco routers use a CLI (Command Line Interface)instead of a GUI interface. You should be familiar with changing and saving paramenters using the Cisco CLI before proceding. The command for changing MMS is ip tcp adjust-mss ‹mss value›.

To set the MTU to 1400 bytes use the following command example:

ip tcp adjust-mss 1360

Additional Notes: If you are experiencing problems related to MTU you should also adjust the MTU on all PCs and Servers connected to the network.
How do I use DrTCP to change the MTU on my NIC?.

Andy Houtz

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by Andy Houtz
last modified: 2004-03-31 14:39:35