Maximum transmission unit (MTU)

1. The MTU command

Router(config-if)# mtu 1480

Defines the maximum size of the payload the pyhiscal interface can encapsulate into an Ethernet frame

If the router receives a frame larger than this value, it cannot process it and immediately drops it.

To verify it:

Router# show int g0/0

2. The ip MTU command

Router(config-if)# ip mtu 1460

Defines the maximum size of an IPv4 packet that can be sent out of that interface

If the router need to send an IP packet that is larger than the ip mtu it will fragment the packet before sending it down to layer 2. (If the packet has the “DF”-Don’t Fragment bit set, the router will drop it and send an ICMP error back to the source)

To verify it:

Router# show ip int g0/0