Looks like this issue applies to the Catalyst 3550 but not the Catalyst 3560.On a 3560 you can use:      monitor session 1 destination interface Fa0/1 encapsulation replicateinstead of:      monitor session 1 destination interface Fa0/1 encapsulation...

I have observed the exact same issue when monitoring 802.1q trunk ports. It seems that the native VLAN of the destination port affects the 802.1q tags that you see. Packets on the source port that are from the same VLAN as the native VLAN of the dest...