Jumbo Frame Test

How can you tell if jumbo frames are working between two devices. Well the easiest way I know is a simple ping test.

Linux:
Success

# ping -s 8000 -M do -c 1 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 8000(8028) bytes of data.
8008 bytes from 192.168.0.1: icmp_seq=1 ttl=255 time=0.555 ms

Failure

# ping -s 9000 -M do -c 1 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 9000(9028) bytes of data.
From 192.168.0.1 icmp_seq=1 Frag needed and DF set (mtu = 9000)

VMware:
Success

# ping -s 8000 -d 10.1.1.101
PING 10.1.1.101 (10.1.1.101): 8000 data bytes
8008 bytes from 10.1.1.101: icmp_seq=0 ttl=64 time=0.410 ms

Failure

# ping -s 9000 -d 10.1.1.101
PING 10.1.1.101 (10.1.1.101): 9000 data bytes
sendto() failed (Message too long)

Even though we set the mtu size to 9000 bytes not all of it can be used by the ICMP payload. The reason for this is because of the 28 bytes of overhead needed for IP plus ICMP. This means the maximum would be 9000 - 28 = 8972 but anything larger than 1472 should prove the point.