Is there a difference between interface MTU and IP MTU ?
Yes, they are different. IP MTU only applies to IP packets.
"Note Changing the MTU value (with the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu command."
you can use the command lookup tool to solve your problem ;)
ip mtu (source: command lookup tool)
To set the maximum transmission unit (MTU) size of IP packets sent on an interface, use the ip mtu command in interface configuration mode. To restore the default MTU size, use the no form of this command.
ip mtu bytes
no ip mtu
MTU in bytes.
Minimum is 128 bytes; maximum depends on the interface medium.
Interface configuration (config-if)
This command was introduced.
This command was integrated into Cisco IOS Release 12.2(33)SRA.
This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.
If an IP packet exceeds the MTU set for the interface, the Cisco IOS software will fragment it.
All devices on a physical medium must have the same protocol MTU in order to operate.
Greetz from Germany ;)