vlan assign - 3com guest vlan

[empirium]

I would like to dynamic assign vlan to switch 3com , and i would like to 
mac based authentication.
In 3com terminology it is called RADA.

Every works fine, freeradius dynamically assign vlan for mac adresses 
includes in users.conf,

But how to configure freeradius  to work in this scenario:
- freeradius check if it has proper mac addres in its users.conf, if it 
has it assign the proper vlan (it works !)
- if it doesnt find a mac in a file it assign vlan for examle 15 (I dont 
know how to configure that in freeradius)

in users.conf I have something:

00-0d-61-1b-6f-15 User-Password  == "00-0d-61-1b-6f-15"
        Tunnel-Medium-Type = 6,
        Tunnel-Private-Group-Id := 101,
        Tunnel-Type = VLAN

Please help
Thanks in advance