If the port you want to monitor is fa0/1 and you would connect the sniffer (Ethereal, Wireshark or Sniffer Pro) to fa0/15 here are the basic commands:
conf terminal
monitor session 1 source interface fa0/1
monitor session 1 destination interface fa0/15
If your switch doesn't support the monitor session syntax, you will need to do the following:
interface FastEthernet0/x
port monitor FastEthernet0/a
port monitor FastEthernet0/b
fa0/x is the port your sniffer is plugged into, 0/a and 0/b would be anyports u want to sniff.
To show monitor session that have configured:
show monitor session local
