Configuring IP Fragmentation-Related Attributes

You can configure the IP datagrams fragmenting attributes using the ip ignore-df-bit and ip mtu commands.

Before you configure IP, you must create the lower-layer interfaces over which IP traffic flows. All IP configurations will be removed from the interface when you issue the no ip interface command in Interface Configuration mode.

• Force the router to ignore the don’t-fragment bit (DF bit) if it is set in the IP packet header for packets on an interface.
  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.

• Set the maximum transmission unit (MTU) size of IP packets sent on an interface. The range is 128–10240.

  Note: Do not configure both Multilink Point-to-Point Protocol (MLPPP) fragmentation (with the ppp fragmentation command) and IP fragmentation of Layer 2 Tunneling Protocol (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.

  host1(config-if)#ip mtu 1000

  Use the no version to restore the default MTU size.