What can cause a route learned through IBGP to be marked as hidden?

A route learned through IBGP can be marked as hidden when the next hop is unreachable. In the context of BGP (Border Gateway Protocol), routes are advertised with an associated next hop IP address, which indicates where to send traffic for that route. If the next hop for a route is not reachable, the route cannot be used effectively by the router, which leads to it being marked as hidden. This is crucial for efficient routing and ensures that routers do not attempt to route traffic to destinations that they cannot reach.

In situations where the next hop is unreachable, the routing decision process will prevent the routing table from using that particular route, effectively hiding it from the routing process, which can lead to suboptimal routing paths if not managed. This mechanism is key for ensuring the overall stability and reliability of the routing infrastructure, as it prevents traffic from being sent down paths that do not lead to the desired destinations.