The use-cache option configures the switch to consult its local authentication cache when RADIUS servers are unreachable. This cache contains session information for supplicants that have previously authenticated successfully. Therefore, devices that are already authorized and have an entry in the cache will be permitted to maintain their access. New devices attempting to connect will not have an entry in the cache, and since the RADIUS server is unavailable to perform a new authentication, they will be denied access. This behavior precisely fulfills the stated requirements.

A. deny: This option blocks all supplicants, including those who were previously authorized, when the RADIUS server is unavailable.

B. vlan-name: This option moves all supplicants to a specified fallback VLAN, altering their network access rather than maintaining their current authorized state.

C. permit: This option allows all supplicants to access the network without authentication, which fails the requirement to deny new access attempts.

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1. Juniper Networks TechLibrary, "Understanding 802.1X Server Fail Fallback": This document details the behavior of the server-fail statement. It explicitly states for the use-cache option: "If the RADIUS servers are unavailable, the switch looks up the supplicant in its local authentication cache... If the supplicant is found in the cache, it is reauthenticated. If the supplicant is not found in the cache, it is denied access to the LAN." This directly supports the correct answer.

2. Juniper Networks TechLibrary, "Configuring 802.1X Server Fail Fallback (CLI Procedure)": This configuration guide describes the server-fail options. Under the use-cache description, it confirms that previously authenticated supplicants found in the cache are reauthenticated, while those not in the cache are denied access, aligning with the scenario's requirements.