Maximum Transmission Unit MTU

MTU specifies the maximum frame size allowed across a medium. For example, on an Ethernet interface on a Cisco router, the default MTU size is set to 1,500 bytes. X.25 can go as low as 128 bytes. Listing 2.4 displays the Ethernet statistics taken from a Cisco router's Ethernet interface, note the MTU size is readily visible.

Listing 2.4 Show interface Ethernet0 command output.

R1>sh interface ethernet0 Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0060.7015.5e4d (bia 0060.7015.5e4d) Internet address is

MTU 1500 bytes,BW 10000 Kbit,DLY 1000 usec,rely 255/255 Encapsulation ARPA, loopback not set, keepalive set (10 sec) ARP type: ARPA, ARP Timeout 04:00:00

The preceding code displays an MTU set to 1,500 bytes. On a Token Ring interface, the MTU is 4,464 bytes, as shown in the Listing 2.5, another command output from a Cisco router.

Listing 2.5 Show interface tokenring0 command output.

R1>sh interface tokenring0 TokenRing0 is up, line protocol is up

Hardware is TMS380, address is 0000.308f.3655 (bia 0000.308f.3655) Internet address is

MTU 4464 bytes, BW 16000 Kbit, DLY 630 usec, rely 255/255 Encapsulation SNAP, loopback not set, keepalive set (10 sec) ARP type: SNAP, ARP Timeout 04:00:00 Ring speed: 16 Mbps

Single ring node, Source Route Transparent Bridge capable Source bridging enabled, srn 2 bn 1 trn 200 (ring group) proxy explorers disabled, spanning explorer enabled

The MTU parameter can be negotiated between end systems when setting up a connection. However, this negotiation will need to be configured in order for it to take place.

Make sure you are familiar with the Ethernet and Token Ring interface display, as shown in Listings 2.4 and 2.5. You should know what each field means and what is relevant and irrelevant. For example, collisions on Token Ring interfaces are meaningless.

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