By default Ethernet interface MTU is 1500. If you want to send a packet size 1600 bytes without fragmentation, you need to increase all non-MPLS interfaces using 'mtu' command to 1600 and MPLS interfaces to 1600 + (4 bytes * number of label). Instead, you can also use 'mpls mtu' command (ie. increase MRU) for all MPLS interfaces as well and keep the MTU the same at 1600.

After playing around on this for a while my final submission is this rule of thumb for the issues are these:

1. MTU interface is the total MTU of the frame which include MPLS MTU plus IP MTU.

2. MPLS MTU is use to ensure that the total MPLS packet size will not exceed the configured MPLS MTU. This situation is important when you have restriction configuring the ETHERNET MTU. And this include IP MTU.

3. IP MTU is the total size require for IP packet before adding MPLS MTU.

Therefore:

ETHERNET MTU = ETHERNET HEADER + TAIL +MPLS MTU + IP MTU

MPLS MTU = MPLS HEADER + TAIL + IP MTU

IP MTU = IP HEADER AND THE REST..

Hope this clears thing..but if you have contrary opinion I will appreciate your input..all is for the learning..

Thanks,

Yeah you are right at that brother..