I have a little C2821 with two GigE interfaces. One is NAT inside, the other is NAT outside. The two interfaces have MTU 9216 (all the hosts here use jumboframes). When I try to use any application that sends large packets, they get dropped at the NAT router. I've confirmed this using ping with large packet sizes from both sides of the NAT.
If I set my host's MTU to 1500, everything works fine. If I remove the NAT, everything works fine.
Has anyone ran into this problem at all? It doesn't make sense as NAT operates at a higher level.