[j-nsp] juniper mpls mtu

[cp]

I hoping someone can provide insight into why by default juniper
calculates mpls mtu at 12 bytes less than the ip mtu?  I've been testing
using l3vpn and it seems that mpls mtu pads 8 bytes to its mpls mtu
reducing the ip mtu the inside packet by 8 bytes. Quick example. So if
mpls mtu is 1508 the ip mtu of the packet inside is 1500. That means the
ip mtu of the mpls interface is 1520. The only reason I can see
additional bytes needed on top of the l3vpn two is for fast reroute
bypass.  I assume it's for the safety factor although I am probably
missing something. Any information is appreciated.

 

-Chip