Multiple NPS servers in one domain

[dblight]

I'm trying to work out a Radius solution for upcoming project but I cannot work out the best way to do it............

 

One AD domain, 6 AD sites.

 

Would you put a Radius server in each site to authenticate its own users to its own Access Points or a centralised Radius server that authenticates all the AP's in every site?

 

I cannot seem to work it out.... my only concern with a centralised server would be if the site link goes down for whatever reason they will lose connection to the server for authentication?

 

Have I answered my own question here? Would multiple Radius servers cause an issue with only one AD domain?

 

Anyone have an experience? Each site is 1 primary school.

 

Its been a long week and my brain is frazzled!