I've set up a mirror port to capture traffic on my 3810, and I've tried mirroring a trunk and just mirroring a single VLAN. In either case, the only traffic that shows up is related to STP:
08:28:49.441655 STP 802.1s, Rapid STP, CIST Flags [Learn, Forward, Agreement], length 102
08:28:49.441694 STP 802.1s, Rapid STP, CIST Flags [Proposal, Learn, Forward, Agreement], length 102
I've not found any documentation that seems to address this. Can anyone point me in the right direction for getting this configured properly?