Final answer:
When a route is not heard for 180 seconds, it is considered to be unreachable or down.
Step-by-step explanation:
When a route is not heard for 180 seconds, it is considered to be unreachable or down.
In computer networking, a route refers to the path that data takes from one network to another. Routing protocols are used to determine the best path for data to travel. If a route is not heard for 180 seconds, it indicates that the network is not able to reach the destination.
For example, in the context of a routing protocol like OSPF (Open Shortest Path First), if a router does not receive updates from a neighbor router for 180 seconds, it will consider that route as not reachable and remove it from the routing table.