A method for determining a link state, comprising: receiving, by a second forwarding device (102, 700, 1000), a third route update packet sent by a first forwarding device (102, 600, 900) and comprising a second attribute indication message, wherein the third route update packet further comprises a third route, a prefix of the third route is an identifier of the first forwarding device, and the second attribute indication message is used to instruct the forwarding device receiving the third route update packet not to advertise the third route to another forwarding device any longer; determining, by the second forwarding device, a target link, and monitoring a state of the target link, wherein the target link is a link from the first forwarding device to the second forwarding device; and sending, by the second forwarding device, a second route update packet to a controller (101, 500, 800) based on the monitored state of the target link, wherein the second route update packet comprises a second route and a target field used to notify the controller of the state of the target link, and a prefix of the second route is the identifier of the first forwarding device; wherein the determining, by the second forwarding device (102, 700, 1000), the target link comprises: determining, by the second forwarding device, a port for receiving the third route update packet; and determining, by the second forwarding device, the target link with the port as an endpoint; or determining, by the second forwarding device, that a link with the port as an endpoint is a member link in a link aggregation group, and determining that the target link is the link aggregation group, wherein the link aggregation group comprises a plurality of member links. receiving, by a second forwarding device (102, 700, 1000), a third route update packet sent by a first forwarding device (102, 600, 900) and comprising a second attribute indication message, wherein the third route update packet further comprises a third route, a prefix of the third route is an identifier of the first forwarding device, and the second attribute indication message is used to instruct the forwarding device receiving the third route update packet not to advertise the third route to another forwarding device any longer; determining, by the second forwarding device, a target link, and monitoring a state of the target link, wherein the target link is a link from the first forwarding device to the second forwarding device; and sending, by the second forwarding device, a second route update packet to a controller (101, 500, 800) based on the monitored state of the target link, wherein the second route update packet comprises a second route and a target field used to notify the controller of the state of the target link, and a prefix of the second route is the identifier of the first forwarding device; wherein the determining, by the second forwarding device (102, 700, 1000), the target link comprises: determining, by the second forwarding device, a port for receiving the third route update packet; and determining, by the second forwarding device, the target link with the port as an endpoint; or determining, by the second forwarding device, that a link with the port as an endpoint is a member link in a link aggregation group, and determining that the target link is the link aggregation group, wherein the link aggregation group comprises a plurality of member links. determining, by the second forwarding device, a port for receiving the third route update packet; and determining, by the second forwarding device, the target link with the port as an endpoint; or determining, by the second forwarding device, that a link with the port as an endpoint is a member link in a link aggregation group, and determining that the target link is the link aggregation group, wherein the link aggregation group comprises a plurality of member links.