MTU ip mtu allowed values
ip mtu command sets the IP MTU size which includes the IP header and the payload. For example, the command
ip mtu 1400 will limit the size of the IP packet to 1400 including the header.
The range of allowed values for this command is from 68 bytes to the interface MTU size. So, if you have set your MTU interface size to 1496 for example, then the allowable range of size for the
ip mtu command will be between 68 and 1496 bytes, as seen below:
R1(config-if)#mtu 1496 R1(config-if)#ip mtu ? <68-1496> MTU (bytes)
A minimum of 68 bytes will never cause an IP packet to be fragmented so much that the header itself would be fragmented.