The problem only occurs when upgrading from 1.0(x) to 2.1(1) versions of the main code. It does not happen between version 1.0(x) and 2.0(1). It also does not happen when changing from version 2.0(1) to 2.1(1). It only occurs when "skipping" from version 1.0(x) to 2.1(1).
If SNA, NetBIOS, or any protocol is using a MTU of greater than 1500 and this event occurs, those sessions will not be able to communicate. The outages can range from occasional dropping of sessions to network wide communication problems.
There are currently two workarounds available.
Go to each port's configuration panel and manually adjust the MTU to the desired level (default 4472).
The second workaround is to upgrade the code in two steps. Download version 2.0(1), reboot, download version 2.1(1), and reboot. This will preserve the port's MTU.
This problem will be fixed in 2.1(2) Catalyst 3900 main code. Upgrade to this code or newer code when it is available.
For More Information
If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods: