To set a port in monitor mode in the C2970G use (in config mode):
# monitor session 1 source interface FastEthernetX/X both # monitor session 1 destination interface FastEthernetX/X
The session number is arbitrary (is used to match source and destination. Instead of both you can use rx or tx.
To show the active monitoring session use:
# show monitor
Remove all the sniffing session with:
# no monitor session all