How do I set the MTU to be 1516 bytes on an ATM subinterface running TR-LANE ?

[admin_2]

I am trying to set the MTU to be 1516 on a Token-Ring LANE Client configured on an ATM subinterface. The port adapter is a PA-A3-OC3MM in a 7204VXR router running IOS 12.2

IOS will only accept two values for MTU-4490 & 9180. See IOS output below:

router(config-if)#int atm1/0.11

router(config-subif)#mtu ?

<64-20940> MTU size in bytes

router(config-subif)#mtu 1516

% Invalid LANE MTU 1516.

Valid sizes are 1500, 4528 and 9218 for Ethernet and 4490 and 9180 for TR.

router(config-subif)#

As I cannot set the MTU to be 1516 (as used throughout a large IBM ATM infrastructure), the LES rejects the clients attempt to join.

I can't see any underlying problem with the protocols which mean only 4490 and 9180 can be used.

Am I missing something? Why can't IOS accept 1516? Surely this problem must have been encountered before?

[#2]

The MTU setting on the interface is the layer 3 packet size. In this case, the MTU is set to 1500, but the largest frame size transmitted on the interface (including layer 2 headers) is 1516. It sounds like you are experiencing another problem at the LES.