Strange MTU size on Cisco Catalyst Express 520
Posted on 2009-05-05
We are currently having problems with a transparent wireless bridge between a UC520 and CE520. After having two of the four wireless units replaced and the problem still existing, we are now digging deeper into the switch/router configuration at either side.
The current set up is a UC520 at one site, two Tranzeo wireless bridges with a switch between them, and then finally a CE520 at another site. The two Cisco devices are set up as a 802.1Q trunk, with 3 VLans (one for each site + management traffic (1 + 2), and voice (100)).
Everything works fine most of the time, but between around 2 to 5 times per day, the wireless link between the sites appears to go down, or at least stop forwarding traffic. This lasts approximately 1 - 3 minutes and appears to be happening at random times. The link restores itself without intervention.
Users notice loss of connectivity to resources, IP Phones often lose registration, spanning-tree events can be seen as root elections take place following the loss of connectivity.
I have noticed a strange MTU setting on the 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
It is my understanding that the MTU should be set to 1500 (and the Cisco Output Interpreter also suggests this is the case). After doing some research, I found a document which says on this device it is not possible to change the MTU "from the default of 1500" (and set system mtu gives invalid command error).
Why is the device reporting 1998 and not 1500 when the document says the (unchangeable) default is 1500?
Is there in fact a way to change this to 1500?
Is this MTU setting likely to be causing the issue?
Any ideas on what else could be the cause if this is not the problem?
Thanks in advance.