Hello Alex. Thank you very much for taking the time to reply.
I understand EX and MX have different chipsets and thus can have a different MTU. I believe my question could be: why would Juniper decide that a low end access/metro device (ex4200) has a chipset that can handle a higher MTU than the chipset of a high end core device (MX960). It doesn't make sense to me, but there probably is a good reason for it and I would like to know.
Unfortunately our newest MPCs are 5, so we won't be able to deliver a 9200+ MTU, I wish there was a workaround. Thank you for the clarification!