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| 4.6 |  | PPP and Layer 2 Considerations for Routed and Routing Protocols |
| 4.6.7 |  | 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.

When an outbound interface has a smaller MTU than the inbound interface, the packet may have to be fragmented. Consider an Ethernet packet of 1500 Bytes that is received on interface fastethernet 0/0, and must be sent out interface serial 0/0 using X.25 encapsulation. X.25 supports MTU values of 576 Bytes.

In this case, the router strips the single Ethernet frame from the Layer 3 packet and reassembles the packet into three X.25 frames. Proper fragment sequencing is maintained in the IP header of the packet. The ID value field contains the same value for each fragment. The Offset field value is unique, and defines which part of the original packet is contained in this fragment. Both sequenced and out of sequence packets are reassembled at the destination device.

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.