Cisco port analyzer SPAN feature

If you are on a Cisco network and need to monitor network traffic or filtering you'll need to setup SPANning on one of your switches. Here is Cisco's Configuring the Catalyst Switched Port Analyzer (SPAN) Feature page.

In a nut shell what is happening is that you are copying traffice from one port (sourse) to another (destination) for monitoring. So lets say you have a firewall on port 0/1 and you want to capture and filter all the web traffic what you would do is plug you monitor into another port say port 0/24. Now you would need to copy all traffic from port 0/1 to 0/24. to do so you'd have to setup a SPAN or monitor session.

#config t
(config)#monitor session 1 source interface fastethernet 0/1
(config)#monitor session 1 destination interface fastethernet 0/24 both
(config)#end

The both at the end of the second command means that this port is bidirectional rx and tx.

Enjoy!