In the Junos OS architecture, the authentication-order statement within the [edit system] hierarchy defines the sequence in which the device attempts to validate user credentials against external and internal databases. The order specified in the exhibit is [ radius tacplus password ] , meaning the device first queries the RADIUS server, followed by the TACACS+ server, and finally the local password database.

A critical distinction in Junos authentication logic is the difference between a server being " unavailable " and a server " rejecting " a user. If an authentication server responds with an explicit access-reject (meaning it received the request but the credentials were invalid), the authentication process terminates immediately to maintain security integrity. However, if the server is unreachable or fails to respond within the configured timeout period (unavailable), the Junos Control Plane automatically falls back to the next method in the defined sequence. In this specific scenario, since the RADIUS server is unavailable, the device will proceed to attempt authentication using the TACACS+ server. If that were also unavailable, it would finally check the local password database. This fallback mechanism ensures that administrators can still access the device even during external server outages, provided they have a local account configured as the final method in the list.