PPP and Layer 2 Considerations for Routed and Routing Protocols
Excessive fragmentation

Fragmentation refers to the reduction of a Layer 3 packet as it is placed in a Layer 2 frame. Each fragment must be less than or equal to the original packet length.

The Maximum Transmission Unit (MTU) refers to the largest Layer 3 packet that can be forwarded out a router or switch interface. Ethernet and serial interfaces have default MTU values of 1500 Bytes.

To change the interface MTU size, two commands can be used. The interface configuration command mtu is used to change the MTU for all Layer 3 protocols. The second interface configuration command, ip mtu is used to set the MTU for the IP protocol.

While relatively straight forward, issues can arise during MTU configuration. If both commands are used on an interface, for example, the ip mtu value takes precedence for IP. However, if mtu is configured after ip mtu, then the IP MTU value is reset to the same value as the MTU value.