Fragmentation

Fragmentation is the process of segmenting a large IP datagram into several smaller pieces. Fragmentation is required when IP must transmit a large packet through a network that transmits smaller packets, or when the MTU size of the other network is smaller.

By default, the system does not fragment the packet if the don't-fragment bit (DF bit) is set in the IP header. You can specify that the system not consider the DF bit before determining whether to fragment a packet.

Note: It is possible that lower-layer protocols may also set the MTU value. If MTU values set in lower layers differ from the one set at the IP layer, the system always uses the MTU lower-layer's value.

    ip ignore-df-bit

• Use to force the system to ignore the DF bit if it is set in the IP packet header for packets on an interface.
• Example

host1(config-if)#ip ignore-df-bit

• Use the no version to restore the default behavior, which is to consider the DF bit before fragmentation.

    ip mtu

• Use to set the MTU size of IP packets sent on an interface.
• The range is 128-10240.
• Example

host1(config-if)#ip mtu 1000

• Use the no version to restore the default MTU size.