Troubleshooting VLANs: How to monitor 802.1q tagged traffic

I work with password protected (PWP) radios that are configured with 802.1q tagging. When I need to troubleshoot a problem I connect directly to the unit but cannot see any information that is tagged with a VLAN. Is there a way to see the tagged traffic when I'm connected to the unit directly?

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If you run show commands for virtual LAN (VLAN) and counters you would have some indication of traffic that is there. However, if you are connected to the unit, you can span all the ports to one port, connect a sniffer at that port and check the traffic. It's a little bit of a cumbersome and involved process, but it will help.

For more information on how to do this, visit HappyRouter.com creator David Davis' tip on three show commands for Cisco admins, or the IT Knowledge Exchange's show command archives or span archives.

This was first published in January 2009

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