Product and Software: This article applies to ArubaOS 3.x and higher.
By default all radius requests use the outgoing VLAN IP address as the source address. This can be changed using the 'ip radius source-interface <loopback / vlan>' command.
This command is a global command and it will change source interface for all Radius servers configured.
Configuring the switch to use loopback as the source address:
(Aruba2400-M) (config) #ip radius source-interface loopback
Configuring the switch to use VLAN as the source address:
(Aruba2400-M) (config) #ip radius source-interface vlan 89
This option is also configurable from WebUI. Browse to Configuration > Security > Authentication, and click Advanced Tab in window on right. Under 'Radius client' there is an option to configure source interface as shown in the following screen.