In a method, a controller sends a first route update packet including a first route to a first forwarding device, so that the first forwarding device sends a third route update packet including a third route to at least one target forwarding device; a second forwarding device receives the third route update packet, determines a target link from the first forwarding device to the second forwarding device, and sends, based on a monitored state of the target link, a second route update packet including a second route and a target field to the controller; and the controller determines the state of the target link based on the target field in the second route update packet.