A server is reachable, when the operational state UP, when a valid response is received within a timeout period which is configurable by the retry parameter on the RADIUS policy level.
A server is treated as not-reachable, when the operational state down, when the following occurs:
a timeout
If a number of consecutive timeouts are encountered for a specific server. This number is configurable by the retry parameter on RADIUS policy level.
a send failed
If a packet cannot be sent to the RADIUS server because the forwarding path toward the RADIUS server is broken (for example, the route is not available, the is interface shutdown, and so on), no retry mechanism is invoked and immediately, the next server in line is used.
A server that is down can only be used again by the RADIUS algorithm after 30 seconds, unless, during these 30 seconds a valid RADIUS reply is received for that server. Then, the server is immediately marked UP again.
The operational state of a server can also be ‟unknown” if the RADIUS application is not aware of the state of the RADIUS server (for example, if the server was previously down but no requests had been sent to the server, therefore, it is not certain yet whether the server is actually reachable).