I spent a good amount of time troubleshooting a L2 link that is provided across two provider networks. The symptoms were packets above 1496bytes being dropped. I used ICMP echo packets to verify this.

# ping x.x.x.x size 1496
Sending 5, 1496-byte ICMP Echos to x.x.x.x, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

# ping x.x.x.x size 1497
Type escape sequence to abort.
Sending 5, 1497-byte ICMP Echos to x.x.x.x, timeout is 2 seconds:
…..
Success rate is 0 percent (0/5)

Our switch on each side has an MTU of 1500 with the provider facing link set to DOT1Q. This means packets being sent on the provider link can be 1500bytes plus the 4byte header for the Q tag. If the provider dosen’t set the link interfaces with an MTU of at least 1504bytes, the traffic will fragment and be dropped.

An easy way to verify the MTU issue on the provider network is to pass traffic in the native VLAN. Since this traffic isn’t carrying the extra 4byte vlan header tag it should pass.

-Justin