A method applicable to a commonest application scenario, the method includes: an AR receives a first packet sent in a unicast manner by a CGN. The first packet includes information for indicating Restart announce, the first packet's source address is the CGN's address, and the first packet's destination address is the AR's address; AR changes the first packet's destination address to an address of a CPE connected to the AR, to obtain a second packet, where the AR and the CPE are connected by using a Layer 2 network, and the CPE's address is a multicast address; and the AR sends the second packet. The method is used for quickly informing, after the CGN restarts or undergoes an active/standby switchover, the CPE that an exception occurs, for example, the CGN restarts or undergoes an active/standby switchover, so that a CGN has relatively small load when recreating NAT mapping.