he SPAN feature configuration commands are similar on the Catalyst 2950 and Catalyst 3550. However, the Catalyst 2950 cannot monitor the VLANs. You can configure the SPAN, as in this example:
C2950#configure terminal
C2950(config)#
C2950(config)#monitor session 1 source interface fastethernet 0/2
!— This configures interface Fast Ethernet 0/2 as source port.
C2950(config)#monitor session 1 destination interface fastethernet 0/3
!— This configures interface Fast Ethernet 0/3 as destination port.
C2950(config)#
The above is all it takes, directly from Cisco help.
Cheers,
Rajesh