FRITZ!Box 3390 Service - Knowledge Base
Manually adjusting the MTU size for DSL connections
The FRITZ!Box does not allow you to adjust the MTU size manually. This is also not necessary because the FRITZ!Box supports the MSS clamping procedure (Maximum Segment Size) that automatically adjusts the size of data packets being sent to a connected network with a smaller MTU. As a result there will be no performance losses due to fragmented or discarded data packets.
Note:The Maximum Transmission Unit (MTU) is the maximum size of user data that can be transferred in a single data packet. A maximum MTU of 1492 bytes is available for DSL connections established by the FRITZ!Box using the PPPoE protocol (Point to Point Protocol over Ethernet). This value may then be further reduced by additional protocols used by the Internet service provider.