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Strange MTU size on Cisco Catalyst Express 520?

I have noticed a (possibly?) strange MTU setting on our Catalyst Express 520, all the other devices are using an MTU of 1500, but the CE520 gives this output :-

Command was: show system mtu

System MTU size is 1998 bytes

System Jumbo MTU size is 1998 bytes

Routing MTU size is 1998 bytes

We have been having some issues with a transparent wireless bridge between a UC520 and this CE520 dropping randomly for approximately one minute, 2 or 3 times a day average.

I ran the Cisco Output Interpreter on interface Gig1 on the CE520 which is the uplink port to the wireless bridge and across to the UC520. The only issue is finds is that the MTU is not the recommended 1500. Could this be causing our problem in any way? All other devices are set to 1500.

After doing some research, I could not find any information about MTU on the CE520, but did find information for the C500. The document says on this device it is not possible to change the MTU "from the default of 1500". I can confirm that the "set system mtu" gives the invalid command error.

Is this a problem or is this behaviour normal? It seems unusual to me but do not want to waste time on it if it is in no way related to our issue. I have checked and the MTU is defininitely (or at least reported to be) 1998 across all interfaces, not just this Gigabit interface.