Here is a link for SPAN configs on 3550 :
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12119ea1/3550scg/swspan.htm#1078974
There are certain limitations which are mentioned in the link above :
You configure SPAN sessions by using parameters that specify the source of network traffic
to monitor. Traffic monitoring in a SPAN session has these restrictions:
You can monitor incoming traffic on a series or range of ports and VLANs.
You can monitor outgoing traffic on a single port; you cannot monitor outgoing traffic on multiple ports.
You cannot monitor outgoing traffic on VLANs.
The command syntax would be like
Switch(config)#monitor session 1 source interface fastethernet 0/2 both
Switch(config)#monitor session 1 destination interface fastethernet 0/8