Hello..
configure the router to forward all the authentication requests of the client to the tacacs server. configure the user options on the tacacs server to assign static ip for the user.
On the router configure :
crypto map clientmap client authentication list userauthen
crypto map clientmap isakmp authorization list groupauthor
aaa authentication login userauthen group tacacs+
aaa authorization network groupauthor group tacacs+
ON the ACS server:
after adding the user, under the client IP address assignment , select the option assign static IP addresses and the put an ip address there.
Try this and let us know !!
