The fallback action comes into effect when connectivity to all RADIUS servers is lost. The operating state of the RADIUS servers changes to either ‟out-of-service” or ‟probing”. There are two configurable fallback actions: accept or user-db. An accept action without force-probing automatically accepts all authentication requests from all subscribers. A user-db action without force-probing uses the local-user-db for subscriber authentication.
Both accept and user-db can be combined with the force-probing command. Force-probing forces the out-of-service server to transition to the probing state immediately, bypassing the hold-down-time timer. Force-probing is a mechanism to promptly restore connectivity to a RADIUS server. A test user is not used to perform a force probe; only actual subscriber authentication is used to test the operating state of the RADIUS server. Probing only occurs when a server is out of service. If all servers are in the probing state, all new incoming authentication requests follow the fallback action immediately.
When probing with an actual subscriber authentication, the 7750 SR only waits for a reply for one timeout interval without any retries. During the wait, the server is in a probing state and no other subscribers are used to probe this server. The subscriber authentication request is not cached when used for probing. Therefore, to trigger authentication again, the subscriber is required to authenticate again with an address request or a data-trigger packet.