Fragmentation refers to the reduction of a Layer 3 packet as it is
placed in a Layer 2 frame. Each fragment must be less than or equal to the
original packet length.
The Maximum Transmission Unit (MTU) refers to the largest Layer 3 packet
that can be forwarded out a router or switch interface.
Ethernet and
serial interfaces have default MTU values of 1500 Bytes.
To change the
interface MTU size, two commands can be used. The interface configuration
command mtu is used to change the MTU for all Layer 3
protocols. The second interface configuration command, ip
mtu is used to set the MTU for the IP protocol.
While
relatively straight forward, issues can arise during MTU configuration. If both
commands are used on an interface, for example, the ip mtu
value takes precedence for IP. However, if mtu is
configured after ip mtu, then the IP MTU value is reset to
the same value as the MTU value.