If you need access to traffic flowing through a network of switches should enable a switch port for us to copy the traffic going the other. This is useful when you need to analyze the behavior of network protocols (STP, CDP, BGP, RIP, etc.) And equipment logs are not enough.
This system will copy all traffic Network mouth you say, so you need to connect a sniffer help you to filter and interpret the protocol traffic and you really need.
Example: We need to monitor the traffic passing through the mouth Gi0 / 1 of our switch. Plug the sniffer on the Gi0/23.
(config) # monitor session 1 source interface GigabitEthernet 0 / 1
(Config) # monitor session 1 destination interface GigabitEthernet 0 / 23
Now only capture the traffic you want and remove the session monitor :
(config) # no monitor session 1
In Cisco equipment can have multiple monitor session depending on the hardware and the version of IOS. Check the documentation to find out what your ability. PD
Obviously ... also see the network traffic can see the Messenger conversations in your office mate;)
These commands are tested on a Cisco3560 switch with IOS 12.2 (52) SE.
There are other brands but with different commands same functionality.
