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Solved
GS316EP: Port mirroring fails to capture traffic between two ports isolated in their own VLAN
Setup: I have two VLANs configured: VLAN 1 (default): most ports, including my normal LAN traffic VLAN 10: exactly two ports (Port 3 and Port 4), used for an isolated point-to-point link betwe...
Wanted to follow up with final results and where I landed, since you both took real time to help troubleshoot this.
StephenB — tried your suggestion of swapping VLAN roles (moving the LAN to the isolated VLAN, moving the WAN link to VLAN 1) to test whether VLAN 1 had privileged status in the mirroring engine. Ran into an unrelated snag getting there (moving the Management VLAN to the isolated segment locked me out of the switch GUI, since that segment has no path back to the internal management subnet) — had to revert before completing a clean test. But I did run one more direct test: added a third plain port to the isolated VLAN (no mirroring involved, just normal VLAN membership) and connected a separate laptop with Wireshark directly. Same result as mirroring — only broadcast/ARP traffic visible, zero unicast traffic between the two isolated-VLAN ports. That's now been confirmed independently across two different capture devices (Security Onion sensor + a separate Wireshark laptop) and two different capture methods (formal port mirroring + plain VLAN bystander port), so I'm confident this isn't a config issue on my end.
schumaku — appreciate the SPAN/RSPAN/ERSPAN context. It sounds like what I'm hitting is a limitation of local SPAN on this switch specifically for a minimal two-port isolated VLAN, without the more advanced RSPAN/ERSPAN capabilities that would be needed to work around it on this class of hardware.
Where I landed: Going with a passive network tap (SharkTap Gigabit Network Sniffer) installed inline between my modem and router's WAN port instead. Since it's electrically passive with no VLAN/switching logic involved, it sidesteps this limitation entirely.
Thanks again for the troubleshooting help — even though we didn't land on a switch-side fix, the process ruled out a lot of possibilities and gave me confidence the tap is the right call rather than a workaround for something I misconfigured. Marking this as resolved on my end.
Port 16. Port 16 is a tagged member of both VLAN 1 and VLAN 10.
You can't do that - the mirroring designation port must provide exact the mirrored port data - so the port where you collect all data mirrored in captive mode must never use any other purpose.
Has anyone successfully mirrored traffic on a similarly isolated VLAN segment on this or a similar NETGEAR model?
That isn't possible on typical small switches with port mirroring only.
Not aware any industry standard software does allow to operate port mirrring adata toghether with actiove production data.
You want to emulate the SPAN port possibilities you might know from the "big boys" e.g. on Cisco enterprise switches, where you can tunnel SPAN data through a dedicated, independant (SPAN) data channel.
Local SPAN:
Copies traffic between ports on the exact same switch.
Remote SPAN (RSPAN):
Sends mirrored data across a dedicated VLAN to a different switch.
Encapsulated Remote SPAN (ERSPAN):
Tunnels mirrored data across Layer 3 IP networks to reach central analysis tools.
Most common aplications require the SPAN (mirrior port) data on a dedicated port, not carrying any addditional prfoducion or other data on top, typical examples are:
Network Troubleshooting:
Engineers use tools like Wireshark on the destination port to inspect live packet flows, pinpoint latency causes, and diagnose dropped connections.
Security Monitoring:
Intrusion Detection Systems (IDS) and security platforms read mirrored feeds to spot malware signatures, scanning behavior, and unauthorized access attempts.
Application Performance Management (APM):
IT teams analyze transaction times and user data streams to ensure software applications run fast and reliably.
Compliance and Forensics:
Auditors record and review data streams to verify that systems follow data privacy rules or to investigate past network breaches.
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