What indicates that a link-state router's neighbor is unreachable?

A link-state router determines that a neighbor is unreachable primarily by the absence of hello packets. In OSPF (Open Shortest Path First), routers maintain adjacency with their neighbors through a process that involves the continual exchange of hello packets. These packets are sent out at regular intervals to ensure that each router is still able to communicate with its neighbors and that the links are operational.

When a router stops receiving hello packets from a neighbor, it suggests that there may be a problem with the link or the neighbor itself may be down. This triggers a process where the router begins to consider the neighbor as unreachable, leading to a reevaluation of the network topology and potentially updating the OSPF database to reflect this change.

In contrast, while timeouts and routing table updates are relevant to OSPF operation, they are consequences of the lack of hello packets rather than direct indicators of unreachability. Establishing a link failing is an initial condition prior to the hello packet exchange and does not solely reflect the ongoing state of the neighbor's reachability. Thus, the lack of hello packets is the primary indication that a neighbor is unreachable, making it the correct outcome in this scenario.