Here is my understanding:
RSPN: Remote switch port analyzer. If you want to capture/sniff the traffic passing through a switchport you can take help of SPAN, but your port to be captured and port where your sniffer is should lie on the same switch.
using RSPAN you can sniff the traffic of a remote switchport i.e. port which is not in local switch, your sniffer and port to be monitored will be on different switches.
you have to assign below configs:
At Source side:
source local and destination remote, Ex.
1#monitor session 2 source int fast0/12
1#monitor session 2 destination remote vlan100
At destination/Sniffer side:
source remote and destination local, Ex.
2#monitor session 4 source remote vlan100
2#monitor session 4 destination int fast0/24
I'm assuming that the sniffer is connected to fast0/24.
Please let us know if I'm able to remove some your confusions.
