If I have two switches connected to each other and one of the switches interfaces are configured for an MTU of 1546, while the other is configured with an MTU of 9216...what problems do I run into. I am also in an MPLS enviroment.
The problem that I'm thinking about is I have a switch that has a system MTU of 1546...my other switch only gives me the option of having the default MTU of 1500 or configuring 'mtu 9216'.
What is important here is the L3 MTU on all L3 interfaces if the devices with MTU 9216 act only as L2 switches you are fine: no one is going to send packets that will use more then 1546 - MPLS stack bytes.
This is the check that you need to do.
If some device has L3 SVI interfaces try to reduce MTU on the L3 interface.
We actually do this way and MPLS VPN services are fine.
We have L2 trunks with MTU 9216 and L3 interfaces with MTU 1520 and is fine.
We use IS-IS and we reduced the clns mtu to 1497 in order to avoid to see input error as giants in some access layer switches.