AAA Authentication query

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I was just trying out some AAA configs on one of my switches. When I attempt the TACAS+ authentication method with the login command, a message comes up saying: The server-group tacas+ is not defined. Please define it.

Can anyone tell me why ? Is this an IOS issue, or do I actually have to be connected up to the server for this work ? I don't have any problems using the radius method.

Cheers.

3550/2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
3550/2(config)#aaa new-model
3550/2(config)#aaa authentication login TEST group tacas+
3550/2(config)#
00:08:33: %AAAA-4-SERVUNDEF: The server-group "tacas+" is not defined. Please define it.
3550/2(config)#aaa authentication login TEST group radius
3550/2(config)#radius host 192.168.100.100
3550/2(config)#line vty 0 4
3550/2(config-line)#login authentication TEST
3550/2(config-line)#^Z