doesn't tcpip.sys need to know ndiswanip's mtu?

Friends,
Hi
In windows 2000, when tcpip.sys first binds to a ethernet NIC, it
will send OID_GEN_MAXIMUM_SIZE to determine the MTU of underlying NIC.
And the IM driver could intercept this OID and modify the MTU. But in
case of NdisWanIp, tcpip.sys send n OID_GEN_MAXIMUT_SIZE or the like.
Then how could tcpip.sys know the MTU of NdisWanIp? Use some hardcoded
value such as 1500,576? I need to modify NdisWanIp's MTU to 1400, and
modifying it in registry is not a good idea since it requires rebooting.
Any advice? Please...