A branch office runs an FTD appliance with two static default routes, the primary pointing to ISP one and a backup to ISP two at a higher metric. The ISP hands addressing down statically with no routing protocol peering. Last night the ISP one gateway stopped answering while the FTD interface stayed up, so traffic blackholed for ten minutes. Which routing change makes the primary route withdraw when its gateway goes silent?
Select an answer to reveal the explanation.
Short Explanation
Static routes don't expire on their own, so you need a route track tied to an SLA monitor that actively pings the gateway. It works like a courier checking the bridge every minute: nobody answers and the route is pulled, letting the backup carry the traffic. Watching the interface only tells you the local cable is still plugged in.
Full Explanation
Static routes have no lifetime of their own; absent route tracking, a silent next hop can leave a blackhole as long as the associated interface remains up. The SLA monitor object sends periodic ICMP echo requests to a monitored address, and the route's Route Tracking field points at that monitor. When no reply arrives within the configured threshold, the FTD withdraws the tracked route from the routing table and the higher-metric backup becomes the active path; the probe continues, allowing the primary route to be reinstalled when the gateway responds again. Relying on a backup metric alone fails because nothing invalidates the primary entry, so the table does not change. Tracking interface state fails because it measures the local physical link, and a link that stays up while the gateway stops answering is exactly what you saw. Requesting a routing protocol fails because this ISP provides no adjacency to peer. Caveat: route tracking applies to IPv4 static routes only, and the monitored target must answer ICMP echo requests. Practical check: block ICMP replies from the gateway briefly and watch the primary route leave the FTD routing table, then confirm it is reinstalled when replies resume.