NAC feature included in 1841 router with security IOS

I'm looking for some guidance, documentation regarding the capabilitys and configuration of NAC on an 1841 router. It looks like it's a software version of NAC that ties to a policy server, maybe an ACS server, or IAS server for example. Is that all it does, in other words, is the capability found mostly on the backend policy server and not the router itself? In that case, what is the router doing, I mean how does it work in relation to NAC? Is it only capable of blocking traffic at layer 3 rather than layer 2 as does 802.1x authentication on a switch of the Clean Access appliance offerred by Cisco?
thank you very much,
Bill

For NAC, the role of a device depends on your network security policy. You can have security applied to any device(s) or you can have it on a policy server which can ensure the security policy. Following link may help you
http://www.cisco.com/application/pdf/en/us/guest/netsol/ns466/c654/cdccont_0900aecd80217e26.pdf

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