Need clarity on MPLS and the various MTUs

Question

Hi All,
I'd like some clarification on MTUs and how they are affected by MPLS. This is my current understanding of a standard IP over Ethernet frame;
Ethernet MTU = 1514
IP MTU = 1500
TCP MSS = 1460
1. If this is correct what happens to the above sizes when MPLS adds one label?
2. Next what happens when MPLS adds a second label for VPN/VFR?
3. Do you have to re-configure switches along the path to accomodate larger Ethernet frames?
4. Also how do the above sizes relate to the MPLS MTU and how will it change depending on one or two labels ?
Next what happens when you try to run MPLS over GRE which I understand has the following characteristics;
Ethernet MTU = 1490 (inside the tunnel)
IP MTU = 1476
TCP MSS = 1436
5. Since you cannot adjust the Ethernet MTU of a Tunnel, as it's dependant on it's carrier interface, what adjustments will be needed in the above to make it work with one or two labels.
6. Last question, the ip tcp adjust-mss can be used to reduce fragmentation of tcp packets in the above environments. Will it work if applied to the PE (Ethernet)interface linking to a P. In other words will the MSS adjustment occur before label switching.
7. In the question above will ip tcp adjust-mss work in both directions e.g. ingress & egress of the MPLS?
If anyone can clarify the above related concepts I would greatly appreciaate it.
Thanks,
Roman.

Answer 1

1- Correct. Assuming you configure "mpls mtu xx", where xx is greater than 1500 (1524 would allow for 6 labels), the ethernet frame would be 4 bytes more.
2- The ethernet frame would be 8 more bytes (4 bytes per label).
3- It depends. Some switches support baby giants (frame slightly larger than 1500 bytes) by default, some others have to be explicitly configured for it and some other don't support it at all.
4- As mentioned above, each mpls label requires 4 additional bytes so "mpls mtu 1524" allows for 6 labels.
5- Why do you want to do MPLS over GRE?
6- If you configure "mpls mtu" and that all switches in your MPLS core support baby giants, you will not have fragmentation. Fragmentation is not good and you probably not want to see in your core.
7- see 6.
Hope this helps,