Im having a series of MTU problems in my network and what the MTU should be for MPLS (to beset optimize fragmentation)

I have a core router, connected to a switch, the router(x86) interface supports jumbo frames, the switch supports 1522 and most hardware upstream (router boards) is at least 1522.

I have several VLANs connecting to the switch from the core router

core ---vlan-->switch---> no vlan ---> backhaul

Any ideas on what my MPLS MTU should be?

If you are not tagging on your backhaul, your packet size will be 1500 bytes

vlan tagging adds 4 bytes
ethernet header is 14 bytes
vpls label adds 4 bytes
mpls label adds 4 bytes

If you have a vlan inside vpls on top of mpls your L2MTU needs to be 1500+4+4+14+4 = 1526 bytes
If you have a vlan inside a vlan your L2MTU needs to be 1500 + 4 + 4 = 1508 bytes

This is covered on the wiki at http://wiki.mikrotik.com/wiki/Manual:Ma ... n_RouterOS

I also covered this in a presentation I did at AU MUM http://mum.mikrotik.com/presentations/A ... thrift.pdf which you may find easier to understand than the wiki

I hope this helps