radius...

why?

radius = remote authentication dial in user service...

the first question one should ask is, do user's actually dial in anymore?

at any rate...

enable aaa new-model as you would for tacacs

aaa new-model

then the radius server
radius-server  host (hostname) (key)

define the 802.1x authentication method

aaa authentication dot1x default group radius

enable 802.1x on the switch

  dot1x system-auth-control

configure each switchport for usage

(config-if)# dot1x port-control {force-authorized | forceunauthorized
| auto}

then sit back and try to figure out why you'd ever use this garbage...