Maximum transmission unit (MTU)

Different physical networks have different maximum frame sizes. Within the different frames, there is a maximum size for the data field. This value is called the maximum transmission unit (MTU), or maximum packet size in TCP/IP terms.

Figure 1 shows the relationship between MTU and frame size.

Figure 1. Relationship of MTU to frame size
The size of the MTU cannot exceed the maximum frame size minus the size of the media access control header.

If an IP datagram is to be sent out onto the network and the size of the datagram is bigger than the MTU, IP fragments the datagram into multiple fragments, so that it fits within the data fields of the frames. If the MTU is larger than the network can support, then the data is lost.

The value of MTU is especially important when bridging is used because of the different network limits. RFC 791 —Internet Protocols states that all IP hosts must be prepared to accept datagrams of up to 576 bytes.

The minimum MTU for IPv6 is 1280. See RFC 2460, Internet Protocol, Version 6 (IPv6) Specification for more information.

You can configure an MTU value by using the MTU parameter on the BEGINROUTES statement.