Section 6.5: MTU and Fragmentation

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.