Switch port mirroring is often used when sniffing traffic to/from a device. Se drawing below.

Switch with port mirroring

Cisco Switch

Example below: Send a copy of traffic on fastethernet 0/1 to fastethernet 0/2. Traffic in both directions are mirrored.

SW1# <input>configure terminal</input>
SW1(config)# <input>monitor session 1 source fastethernet0/1</input> 
SW1(config)# <input>monitor session 1 destination fastethernet 0/2</input>

Remove mirror from session 1.

SW1# <input>CONFIGURE TERMINAL</input>
SW1(config)# <input>no MONITOR SESSION 1</input>

Zyxel switch

Copy traffic from port 1,4,5 and 6 to port 3

Switch(config)# <input>mirror-port</input>
Switch(config)# <input>mirror-port 3</input>
Switch(config)# <input>interface port-channel 1,4-6</input>
Switch(config-interface)# <input>mirror</input>
Switch(config-interface)# <input>mirror dir egress</input>

Remove mirror again

Switch(config)# <input>no mirror-port</input>