Having a radius server is a great way to securely manage your router/switch's passwords and keep things consistant on your network.
Router/Switch Side config:
Router(config)#username yourUser privilege 15
Router(config)#aaa new-model
Router(config)#aaa authentication login default local group radius
Router(config)#aaa authentication login telnet group radius line enable
Router(config)#radius-server host ServerIPAddress auth-port portNumber acct-port portNumber
Router(config)#radius-server retransmit 3
Router(config)#radius-server key YouRadiusKey
And that Should get you through the basics of setting up RADIUS authentication on your Cisco Router/Switch
Router/Switch Side config:
Router(config)#username yourUser privilege 15
Router(config)#aaa new-model
Router(config)#aaa authentication login default local group radius
Router(config)#aaa authentication login telnet group radius line enable
Router(config)#radius-server host ServerIPAddress auth-port portNumber acct-port portNumber
Router(config)#radius-server retransmit 3
Router(config)#radius-server key YouRadiusKey
And that Should get you through the basics of setting up RADIUS authentication on your Cisco Router/Switch