[Linux-cluster] fencing?

[Jason]

ok, so I have GFS installed and have the modules loaded, seems the next step is to set up "fencing"
and looking at the list of agents at,
http://www.redhat.com/docs/manuals/csgfs/admin-guide/s1-fence-methods.html
im kinda confused. I thought that the idea behind fencing was to allow one node (i have 2) to take 
down/reboot the other node after it has detected some sort of fault in the other node.

so I understand APC Network Power Switch and WTI Network Power Switch
but since I dont have any of those installed, Im going down the list and see
GNBD and xCAT as options. I dont understand what these software packages are supposed to be doing 
in relation to being a fencing agent. Can I use one of these options reliably without having a 
hardware power switch? I guess in short, the docs I have read dont quite explain how fencing is 
supposed to work with GFS.

regards,
Jason