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MPLS label stack depth and what MTU required?

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

1 ACCEPTED SOLUTION

Accepted Solutions

Re: MPLS label stack depth and what MTU required?

Hi Chris,

Actually the layer 2 header is 18 byte Ethernet header and Cyclic Redundancy Check (CRC) trailer, excluding the 4 bytes for the dot1q, and in this way you have your math done (1500 + 18 + 4 + 8).

BR,

Mohammed Mahmoud.

1 REPLY

Re: MPLS label stack depth and what MTU required?

Hi Chris,

Actually the layer 2 header is 18 byte Ethernet header and Cyclic Redundancy Check (CRC) trailer, excluding the 4 bytes for the dot1q, and in this way you have your math done (1500 + 18 + 4 + 8).

BR,

Mohammed Mahmoud.

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