TCP/IP defines a maximum length for an IP packet. The term used to describe that maximum length is
maximum transmission unit (MTU). The MTU varies based on configuration and the interface's
characteristics. By default, a computer calculates an interface's MTU based on the maximum size of the data
portion of the data-link frame. IP hosts, including routers, cannot forward a packet out an interface if the
packet is longer than the MTU. Therefore, if a router's interface MTU is smaller than a packet that must be
forwarded, the router fragments the packet into smaller packets, each of which is less than or equal to the
MTU value.
The fragmented packets are reassembled by the endpoint host. The IP header contains fields that are used for
reassembling the fragments. This includes an ID value that is the same in each fragmented packet, as well as
an offset value that defines which part of the original packet is held in each fragment. Therefore fragmented
packets can be reassembled in the correct order.
Two configuration commands can be used to change the IP MTU size on an interface. These are:
• mtu, which sets the MTU for all Layer 3 protocols; and
• ip mtu, which sets the MTU for IP only.
If both mtu and ip mtu are configured on an interface, the ip mtu setting takes precedence. However, if the
mtu command is configured after ip mtu is configured, the ip mtu value is reset to the mtu value.