He means, do you have to buy a second copy at full price, or is their a way to get it at reduced price if its going to run as a backup.
To answer your question -- to remain legal you have to buy a second copy. We went through this about a year ago, and what it comes down to is Cisco wants you to buy another copy of it. Ridiculous in my opinion. However, having only one ACS server was an unacceptable risk in my opinion as it was controlling so much stuff now and we could not afford for it to go down..
Brian
