If check-gateway is enabled for an ECMP route and one gateway is unreachable, what happens?

When check-gateway is enabled for Equal-Cost Multi-Path (ECMP) routing, the router periodically checks the reachability of the defined gateways. If one of the gateways is found to be unreachable, it is effectively ignored in the routing process. The remaining reachable gateways will still be utilized for forwarding packets, allowing for continuous data transmission even if one path is down.

This setup is designed to enhance network reliability and efficiency by dynamically responding to the status of each gateway. Thus, while one gateway may be down, the traffic can continue to be routed through other active gateways, ensuring minimal disruption in network performance. The key aspect of this feature is that it allows for better load balancing and redundancy by leveraging only the reachable gateways without requiring manual intervention or reconfiguration.