Forwarding labeled packets is quite different from forwarding IP packets in that not only is the IP lookup replaced with a lookup of the label in the label forwarding information base (LFIB) but different label operations are also possible.
All IPv4 packets have one or more labels. This does imply that the labeled packets are slightly bigger than the IP packets, because for every label, four bytes are added to the packet. So, if n is the number of labels, n * 4 bytes are added to the size of the packet when the packet is labeled.
Knowing how many labels are in play - will help us understand the best MTU to utilize - especially since PPPoE already is using some of the MTU as well.
Let's keep PPPoE out of the mix for just a minute when doing this example
If you know that all packets that are sent on the link have a maximum of two labels and the MTU is 1500 bytes, you can set the MPLS MTU to 1508 (1500 + 2 * 4). Thus, all labeled packets of size 1508 bytes (labels included) can be sent on the link without fragmenting them IF you are able to utilize Jumbo Frames... (which chances are your provider and the Internet will NOT allow)
Of course - once we add PPPoE to the mix- we need to account for the MTU change as well. PPPoE (PPP over Ethernet) bridging which requires an extra 8 byte header for PPP and then sends the packet over 1,500 byte Ethernet, so the packets sent in the PPP layer can be at most 1,492 bytes BEFORE allotting for any labels.
This should help you figure out the Proper MTU based upon the labels and having PPPoE in the mix.
One of the Better talks I have listened to is from Tomas Kirnak his slides from a MUM presentation done in 2013 are located here: http://mum.mikrotik.com/presentations/US13/kirnak.pdf
and you can watch that here: http://tiktube.com/video/KHhE3aEKdDoDEJ ... sDlIoFqoq=
Glenn Kelley | MCTNA, MTCWE, MTCTCE, RHCE, RHCSS
USA Based 24x7x365 Mikrotik, Juniper, Ubiquiti TAC & WISP / ISP Blind Label Support Call Center