Maximum Transmission Unit, or MTU, is an often overlooked aspect of a network's configuration. Each network interface has its own MTU that defines the largest packet that it can send. In most networks, the MTU assumes the widely known standard value for Ethernet, 1500 bytes, and rarely results in problems. However, when MTU problems do arise, they generate a range of problems from inefficient data transfers, that rob users of the network's full capacity, to crippling packet loss that can bring networks to their knees. Worst, MTU misalignments are very difficult to identify and isolate.

They can arise anywhere but are most often found where two different Layer 2 protocols converge, since each protocol has a different standard MTU. And even on homogeneous Ethernet networks, problems can arise, when network interfaces are set to values other than the maximum allowable value.

When Internet Protocol (IP) is used to transfer data across a path, data is encapsulated into packets before it leaves the physical interface. If the application is sending a small chunk of data, for example 500 bytes, it will usually be sent within a single packet. However, when the application must send a larger chunk of data, the data is typically distributed over several packets, or fragmented. The MTU of the media determines the maximum size of the packets that can be transmitted without fragmentation.

Network links that have properly configured MTUs are more efficient. Typically the Internet operates with an MTU of 1,500 bytes, the Ethernet standard, however other values are acceptable. Mixing of different MTU values within one network path is also acceptable provided that all components within a network path share similar rules regarding conflicting MTUs. As a general rule, fragmentation should be avoided at all costs.
The maximum packet size allowed on a network without fragmentation is based on the type of network connections involved.