Feature Description - IP Services 01

Maximum transmission unit (MTU) defines the largest size of packets that an interface can sent without the need to fragment. IP packets larger than the MTU are fragmented before they are sent out of an interface.

MTU is used to limit frame lengths on the link layer. In fact, devices of different vendors and even different product models of the same vendor have different MTU definitions.

Take the Ethernet as an example.

Figure 7-1 A complete Ethernet frame (in bytes)

• In some devices, the MTU configured on the Ethernet interface indicates the largest size of the IP datagram of the Ethernet frame, that is, the MTU is layer3 definition, named as IP MTU.

• In some devices, the MTU = Data payload + Destination MAC + Source MAC + Length, that is, the MTU = IP MTU + 14Bytes.
• In other devices, the MTU = Data payload + Destination MAC + Source MAC + Length + CRC, that is, MTU = IP MTU + 18Bytes.

In NE, the MTU is a layer 3 definition. As shown in the Figure 1, the MTU indicates the largest size of the IP header + IP payload. If the MTU configured on an interface of Ethernet type is 1500 bytes, the packet will not be fragmented if the total length of the IP header and IP payload is not larger than 1500 bytes.

Figure 7-2 The MTU definition in NE

The interfaces of NE support an MTU between 46 and 9600 bytes. Each interface supports a default MTU.