Why are we not able to ping more than 1472 packets when df-flag set while using default MTU on the network?
On Aruba Mobility controller we have an option to use extended ping. When the MTU on the network is set to default MTU(1500) we will not be able to ping packet size set to more than 1472 when df-flag (do not fragment) is used.
The reason for this is that along with the default size, you have to account for 8 bytes being used for the ICMP header and another 20 bytes for the IP.
This comes out to:
1500 - 8 - 20 = 1472
As such, 1472 bytes is the largest payload you can set where you have do fragment turned off.
(Aruba-Master7240) #ping 10.17.169.150 df-flag packet-size 1472 count 20 Press 'q' to abort. Sending 20, 1472-byte ICMP Echos to 10.17.169.150, timeout is 2 seconds: !!!!!!!!!!!!!!!!!!!! Success rate is 100 percent (20/20), round-trip min/avg/max = 0.331/0.8275/5.938 ms (Aruba-Master7240) #ping 10.17.169.150 df-flag packet-size 1473 count 20 Press 'q' to abort. Sending 20, 1473-byte ICMP Echos to 10.17.169.150, timeout is 2 seconds: .................... Success rate is 0 percent (0/20)