The ip radius source-interface command is needed for this configuration because it forces RADIUS to use the IP address of a specified interface for all outgoing RADIUS packets. This ensures that the RADIUS server can identify the source of the requests and apply the appropriate policies. If this command is not used, RADIUS will use the IP address of the interface that is closest to the destination, which may not match the configured NAS IP on the RADIUS server. This can cause the RADIUS server to reject the requests as unauthorized12. References: * 1: RADIUS source interface - Cisco Community * 2: RADIUS Commands - Cisco