You issue the ping 192.168.1.1 size 1600 command on a Cisco device that is configured with the default system MTU. All pings succeed. Which of the following is most likely true? (Select the best answer.)

Explanation:

Most likely, the pings were sent to the destination in a fragmented fashion because the IP version 4 (IPv4) donotfragment bit, or DF bit, has not been set in this scenario. By default, packet fragmentation is used to enable oversized packets to traverse the network in chunks that are smaller than the configured maximum transmission unit (MTU.) Enabling the DF bit configures the ping command to attempt to send packets of a given size without fragmentation. By repeatedly pinging a destination device with smaller and smaller datagram sizes, you can determine the MTU.

The ping command supports the ability to modify the size of the datagram that it transmits as well as the ability to enable the DF bit, which is disabled by default. You can configure extended ping features either by issuing the ping command without parameters, which causes the ping command to display a series of configuration prompts, or by specifying parameters on the command line along with the ping command. For example, the ping 192.168.1.1 size 1500 dfbit command configures an extended ping with a destination IP address of 192.168.1.1, a datagram size of 1,500 bytes, and an enabled DF bit. On a connection with an MTU of 1,500 bytes, this ping succeeds, as shown in the following output:

Based on the output, you can determine that the ping succeeded. You can also determine that the DF bit is, indeed, enabled. However, issuing the ping 192.168.1.1 size 1501 dfbit command on the same device results in a failure, as shown in the following output:

In the output above, issuing the ping 192.168.1.1 size 1501 dfbit command results in a ping failure because the MTU is configured to 1,500 bytes and the DF bit is set. If you were to issue the same command without the dfbit parameter, the ping would succeed because the ping command is allowing the datagram to be fragmented, as shown in the following output:

The interface is not configured with an MTU value of at least 1,600 in this scenario. In addition, the datagram size specified is not below the default MTU value on a Cisco device. By default, a Cisco device has a system MTU of 1,500 bytes. In this scenario, you have issued the ping 192.168.1.1 size 1600 command on a device that is configured with the system default MTU.

The DF bit has not been enabled in this scenario. In order to enable the DF bit, you should issue the ping command with the dfbit parameter.