you can use a different MTU for ISIS PDUs by setting the CLNS mtu using interface command
disabling hello-padding should also be considered under router isis process.
In the past there have been issues with large MTU like 9000 bytes that should be solved in more modern IOS versions.
When testing, test the adjacency setup and also the capability to recover after a fault (rebuilding of ISIS adjacency).
As this was reported as troublesome ( ISIS adjacency rebuilding).
Be also aware that implementation of point-to-point network type concept is different in IOS and in IOS XR, with the first one still using separate L1 and L2 LAN hellos (at least in some IOS versions of C7600/C6500) and IOS XR moving to L1L2 serial hello in the logical point-to-point.
Hope to help
Thanks for the suggestions.
I have got 7206 VXR at both ends. Below is how the interfaces are configured.
One of my Customer needs to change to a higher MTU size but ISIS stops working whenever I try to increase the MTU size. It works if i decrease the mtu. For testing i used "clns mtu 1500"and ISIS went down but when I used "clns mtu 90", it works fine.
you have GRE Tunnel interfaces
in the past in a scenario like yours (GRE Tunnels between CE routers in an MPLS L3 VPN in my case) I used
clns mtu 1460
no isis hello-padding
and this worked fine for us.
It looked like it was enough to accomodate GRE overhead to have a working ISIS adjacency over the GRE tunnel.
Hope to help