Overview

The MTU (maximum transmission unit) is the maximum size of an IP packet that can be transmitted without fragmentation. If your network device requires a specific MTU (as, for example, some types of modems do), you can define the MTU in the Kerio Control interface.

Note: the same advanced menu UI allows to override MAC address for the specific interface.

Solution

1. In the administration interface, go to Interfaces.
2. Double-click on the interface.
3. In the Interface Properties dialog box, click on Advanced.
   
   
4. In the Advanced Interface Properties dialog box, select Override MTU and type the value in bytes. For ADSL, the MTU value is 1492 or less. See the documentation for your ADSL modem.
5. (Optional) Define custom MAC address in the Override MAC address field.
6. Click on OK twice.

Testing

The interface now uses the newly configured MTU. The config logs show the following Update Interfaces entry:

[31/Aug/2020 12:28:00] Admin - UPDATE Interfaces_v2 SET MtuOverride=1500 WHERE Id=71fab59e-x-x-x-x [Name: Ethernet, Original values: MtuOverride=0]