Config
C2950#configure terminal C2950(config)# C2950(config)#monitor session 1 source interface fastethernet 0/2 C2950(config)#monitor session 1 destination interface fastethernet 0/3
Check Status
C2950(config)# Type : Local Session Source Ports : Both : Fa1/0/2 Destination Ports : Fa1/0/3 Encapsulation : Native Ingress : Disabled
Remove Config
C2950#configure terminal C2950(config)# C2950(config)#no monitor session 1
http://wiki.netmon.ca/index.php/How_to_set_up_port_spanning_on_Cisco_devices