Tunnel interface ip mtu vs mpls mtu commands

[kevin.r719]

hey,

so if you have an encrypted tunnel and the tunnel ip mtu is 1400, how do you set the

mpls mtu

command if you expect 2 labels max? do you make it the same as the tunnel mtu? Or how do these commands work exactly?

[M02@rt37 · Accepted solution]

Hello @kevin.r719 

Calculate the MPLS MTU by subtracting the MPLS label stack overhead from the IP MTU. So, if IP MTU is 1400 and the MPLS overhead for two labels is, i.e.8 bytes, then the MPLS MTU might be set to 1392.

[kevin.r719]

Thanks. I can't really find the documentation on this so essentially it's the max mtu before the mpls labels

[M02@rt37]

@kevin.r719 

Yes, the MPLS MTU is essentially the maximum size of the packet before MPLS labels are added. In your case, with a tunnel IP MTU of 1400 and considering 2 MPLS labels with an overhead of 8 bytes, the MPLS MTU would be 1392. This ensures that the packets can accommodate the MPLS label information within the specified maximum size. 

[MHM Cisco World]

I will test this case in lab if you want.

Thanks 

MHM

[kevin.r719]

that would be cool. constraints

the ip mtu is 1400

using ipsec gre's 

the maximum labels 2

[MHM Cisco World]

Yes I prefer lab for some case.

I Will run lab and update you tonight 

MHM