Internet Protocol (IP) Basics. IP MTU
Each Network Interface Layer technology imposes a maximum-sized frame that can be sent. The maximum-sized frame consists of the framing header and trailer, and a payload. The maximum size of a frame for a given Network Interface Layer technology is called the maximum transmission unit (MTU). For an IP packet, the Network Interface Layer payload is an IP datagram. Therefore, the maximum-sized payload becomes the maximum-sized IP datagram. This is known as the IP MTU.
Table 4-1 lists the IP MTUs for the various Network Interface Layer technologies that Chapters 1 and 2 discuss.
Table 4-1 IP MTUs for Common Network Interface Layer Technologies Network Interface Layer Technology IP MTU
Ethernet (Ethernet II encapsulation) 1500
Ethernet (IEEE 802.3 SNAP encapsulation) 1492
Token Ring (4 and 16 Mbps) varies based on token holding time
Frame Relay 1600
ATM (Classical IP over ATM) 9180
Minimum MTU 576
In an environment with mixed Network Interface Layer Protocols, fragmentation can occur when crossing a router from a link with a higher IP MTU to a link with a lower IP MTU. IP fragmentation is discussed in more detail in the “Fragmentation” section of this chapter.