mtu
Overview
Maximum transmission unit for an interface.
[no] mtu if_name bytesshow mtu
Syntax
if_name The internal or external network interface name. bytes Number of bytes. Range is from 64 to 65,535 bytes, depending on the type of network connected to the interface.
Usage
Set the size of data sent on a connection. Data larger than the MTU value is fragmented before being sent.
Firewalls that support the IP Path MTU Discovery mechanism, as defined in RFC 1191. allow a host to dynamically discover and cope with differences in the maximum allowable maximum transmission unit (MTU) size of the various links along the path.
Sometimes a firewall is unable to forward a datagram because it requires that the packet be fragmented, but the "don't fragment" (DF) bit is set. The network software sends a message to the sending host, alerting it to the problem. The host will have to fragment packets for the destination so that they fit the smallest packet size of all the links along the path.
For Ethernet interfaces, the default MTU is 1500 bytes in a block, which is also the maximum. This value is sufficient for most applications, but you can pick a lower number if network conditions warrant it.
For Token Ring and FDDI, the default is 8192 bytes.
The no mtu command resets the MTU block size to 1500 for Ethernet interfaces and 8192 for Token Ring. The show mtu command displays the current block size. The show interface command also shows the MTU value.
Configuration mode.
Examples
The following example shows use of the mtu command for use with Token Ring and Ethernet:
interface token-ring0 16mbps
interface ethernet0 auto
mtu inside 8192
show mtu
mtu outside 1500
mtu inside 8192