[c-nsp] MPLS Label stack depth and MTU

[Christopher Hunt]

Greetings,
   Given an all-cisco Layer3 MPLS VPN (RFC 2547bis) network what are the 
maximum MPLS label stack depth and Path MTU required?

CE1--a--PE1--b--P1--c--P2--d--PE2--e-CE2

    My understanding is there is one 4 byte label for the MPLS VPN 
(MP-BGP label) plus one for MPLS transport (which is switched out by 
each P, then popped by PE2. If I want to deliver an edge MTU (at points 
a and e) of 1500 i expect to need a core MTU (at points b,c,e) of 1526 
bytes (edge MTU 1500 + transport header (18 bytes for 802.1Q Vlan 
tagging) + 8 bytes for the 2 MPLS labels.  I do have a core MTU (at 
points b,c,d) of 1526 but I'm only seeing and edge MTU of 1496.  Can 
anyone confirm or deny whether 3 MPLS headers might be in use here?  If 
only 2, where are my other 4 bytes going?   Thanks!


-Chris