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

Note: Lower-layer protocols can also set the MTU value. If MTU values set in lower layers differ from the one set at the IP layer, the router always uses the MTU lower-layer value.

ip ignore-df-bit

Use to force the router 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.
See ip ignore-df-bit

ip mtu

Use to set the MTU size of IP packets sent on an interface.
The range is 128–10240.
Do not configure both MLPPP fragmentation (with the ppp fragmentation command) and IP fragmentation of L2TP packets (with the ip mtu command) on the same interface. Instead, you must choose only one of the fragmentation configurations by setting it to the necessary value and set the other fragmentation configuration to the maximum allowable value.
Example
host1(config-if)#ip mtu 1000
Use the no version to restore the default MTU size.
See ip mtu