Csico 3640 => MTU/ISIS/MPLS

On an Etherenet or Fast Ethernet interface the maximum MTU size is 1500. The maximum MPLS MTU size is 1476 because of the additional IP and MPLS header (20 byte IP header and 4-byte MPLS header.) If this is a serial interface you can use larger MTU sizes, but it must be set to the same value on BOTH sides. If one side is set to 1500 and the othe is set ot 1548, the side with an MTU of 1500 will drop any packet that it receives that is larger than that value.

OSPF has a special interface configuration command

ip ospf mtu-ignore

without it OSPF doesn't work if mtu of one device doesn't match mtu on other devices. I guess IS-IS might have something similar.

But mpls mtu 1548 is not enough. You need your hardware to support lager packets too. And here we come to the bad thing: FastEthernet interfaces on 3640 doesn't allow packets to be larger than 1500 bytes.

Good news:

mpls mtu N

automatically increases interface mtu. Just make sure, your switch supports large mtu.