GS316EP: Port mirroring fails to capture traffic between two ports isolated in their own VLAN

Thank you both for the detailed responses — really appreciate the expertise.

StephenB — you're right that mirroring both Port 3 and Port 4 was redundant since it's a single pass-through link. I've simplified to just mirroring Port 3.

schumaku — good catch on Port 16 needing to be a clean, single-purpose mirror destination. I reverted it to a plain untagged member of VLAN 1 only (its original, working configuration), removed it entirely from VLAN 10, and confirmed via the PVID table that it's back to a proper dedicated mirror port with no other VLAN membership.

Unfortunately, even with this cleaner configuration, the result is unchanged: mirroring from Port 1 (my normal LAN VLAN) to Port 16 works perfectly, but mirroring from Port 3 (the isolated two-port VLAN carrying my modem-to-router link) still captures zero packets on the destination — confirmed via tcpdump on the receiving end, both with and without VLAN tag filtering, over multiple 60+ second active-traffic test windows.

For what it's worth, I also confirmed via port statistics that Port 3 has substantial live traffic flowing through it the entire time — so the switch is definitely seeing and forwarding the traffic normally, it's just not reaching the mirror destination.

At this point I've tested: basic mirror config, VLAN membership on source and destination, tagged vs. untagged handling, and port count in the isolated VLAN (2 vs. 3 members) — all with the same negative result. 

If either of you has seen this specific pattern before, or knows of a firmware setting that might affect it, I'd love to hear it — otherwise I'll likely move forward with a passive network tap for this particular link, since the mirroring approach doesn't seem to be working here.

Thanks again for taking the time.