Hi, we have an ISP provided private MPLS network. We've left the MTU on the CE MPLS interface as default ie 1500.
Reading around these forums, it seems that you can increase this to prevent fragmentation?
Should I do this or leave it at it's default? Everything seems to be working fine although if we can squeeze a little bit more performance out of it by avoiding fragmentation, the better.
as everything in life, it depends; you should leave it, your question is for CE, and the MPLS MTU applies on the MPLS cloud; assuming your CE is using 1500 on customer side and MPLS on ISP side, the ISP side interface should accommodate extra bytes for MPLS header without your change.
Now, some devices support larger MTU like 9200, but if you want to go that way it's a change for the ISP, and then you should adjust your enterprise side accordingly to take advantage of the new MTU, which will be a non standard MTU on your side, but it works for some environments as data centers.
briefly, you don't have to change it.
Whats the MTU supported by the ISP?
You may increase your MPLS MTU for MPLS packets to a value of (ISP MTU - number of labels you are using X 4) as expected. You may first need to change interface MTU.
But the mpls mtu command effects is different for some platforms;