can be forwarded out an interface. The maximum MTU value allowed is based on the data link
protocol; essentially, the maximum size of the data portion of the data link frame (where the
packet is placed) is the maximum setting for the MTU on an interface. The default MTU value
is 1500.
performed by the router. Fragmentation is the process of simply breaking the packet into
smaller packets, each of which is less than or equal to the MTU value. For instance, consider
Figure 5-29, with a point-to-point serial link whose MTU has been lowered to 1000.
the Ethernet header but could not forward the packet because it was 1500 bytes and the HDLC
link supported only an MTU of 1000. So, LA fragmented the original packet into two packets.
After forwarding the two packets, Boston receives the packets and does not reassemble them--
reassembly is done by the endpoint host, which in this case is Clemens.
The IP header includes an ID value that is the same in each fragmented packet, as well as an
offset value that defines which part of the original packet is held in each fragment. So,
fragmented packets arriving out of order can be identified as part of the same original packet
and can be reassembled into the correct order using the offset field in each fragment.
interface subcommand and the ip mtu interface subcommand. The mtu command sets the
MTU for all Layer 3 protocols; unless there is a need to vary the setting per Layer 3 protocol,