Ip mtu command

From NetworkCommands

A command available on most Cisco router platforms; used to configure the maximum transmission unit (MTU) of IP packets sent on the interface.

Usage Syntax

ip mtu <size_in_bytes>

The minimum value is 128 bytes, the maximum is media dependent.

Usage Examples

ip mtu 1200 - set the IP MTU value to 1200 bytes

no ip mtu - restore the IP MTU to the default value

Usage Notes

If an outbound packet exceeds the IP MTU, it will be fragmented. If the Don't Fragment (DF) bit is set, the packet will be dropped. Requires Confirmation

(To ensure this doesn't happen use the ip tcp adjust-mss command on relevant interfaces.)

You cannot set the IP MTU value to a value higher than the layer 2 MTU.

Remember that the IP MTU includes the TCP and IP headers of 20 bytes each so the TCP/IP Maximum Segment Size (or date payload) is actually 40 bytes less than the IP MTU value.

If the IP MTU and layer 2 MTU value have the same value, adjusting the layer 2 MTU value automatically adjusts the IP MTU value.

If the layer 2 MTU value is reduced to a value below the IP MTU value, the IP MTU will be lowered to match it.

Related Commands

You can use the ip tcp adjust-mss command to adjust the value of the MSS option field in TCP SYN packets that pass through interfaces configured with this command. This allows you to set the MSS to a suitable, working value regardless of actual host settings.

You can use the crypto ipsec df-bit command to clear the Don't Fragment (DF) bit in packets passing through interfaces configured with IPSEC tunnels.

Related Articles

More information on the Ethernet & IP MTU.