What is claimed is:1. An authentication method, wherein the method comprises:receiving, by a mobility management network device, first link information, wherein the first link information is used to represent an access location of a residential gateway;obtaining, by the mobility management network device, subscription information of the residential gateway, wherein the subscription information of the residential gateway comprises second link information of the residential gateway and second virtual interface information of the residential gateway, the second link information is used to represent a location of the residential gateway, and the second virtual interface information is used to represent a service type of the residential gateway; andverifying, by the mobility management network device, validity of the access location of the residential gateway based on the first link information and the second link information;receiving, by the mobility management network device, first virtual interface information, wherein the first virtual interface information is used to represent a current service type of the residential gateway; andverifying, by the mobility management network device, validity of a current service of the residential gateway based on the first virtual interface information and the second virtual interface information; andsending, by the mobility management network device, a non-access stratum security mode command (NAS SMC) request message to the residential gateway, wherein the NAS SMC request message comprises the first virtual interface information, and the first virtual interface information is used by the residential gateway to verify whether the first virtual interface information is modified on an air interface.2. The method according to claim 1, wherein there are a plurality of pieces of second link information; andthe verifying, by the mobility management network device, validity of the access location of the residential gateway based on the first link information and the second link information comprises:if the first link information matches any one of the plurality of pieces of second link information, determining, by the mobility management network device, that the access location of the residential gateway is valid.3. The method according to claim 1, wherein there are a plurality of pieces of second virtual interface information; andthe verifying, by the mobility management network device, validity of a current service of the residential gateway based on the first virtual interface information and the second virtual interface information comprises:if the first virtual interface information matches any one of the plurality of pieces of second virtual interface information, determining, by the mobility management network device, that the current service of the residential gateway is valid.4. A mobility management network device, wherein the mobility management network device comprises at least one processor configured to execute instructions stored in a memory, wherein the instructions instruct the at least one processor to:receive first link information, wherein the first link information is used to represent an access location of a residential gateway;obtain subscription information of the residential gateway, wherein the subscription information of the residential gateway comprises second link information of the residential gateway and second virtual interface information of the residential gateway, the second link information is used to represent a location of the residential gateway, and the second virtual interface information is used to represent a service type of the residential gateway;verify validity of the access location of the residential gateway based on the first link information and the second link information; andreceive first virtual interface information, wherein the first virtual interface information is used to represent a current service type of the residential gateway;verify validity of a current service of the residential gateway based on the first virtual interface information and the second virtual interface information; andsend a non-access stratum security mode command (NAS SMC) request message to the residential gateway, wherein the NAS SMC request message comprises the first virtual interface information, and the first virtual interface information is used by the residential gateway to verify whether the first virtual interface information is modified on an air interface.5. The mobility management network device according to claim 4, wherein there are a plurality of pieces of second link information; andthe instructions instruct the at least one processor to verify validity of the access location of the residential gateway based on the first link information and the second link information comprises:if the first link information matches any one of the plurality of pieces of second link information, determine that the access location of the residential gateway is valid.6. The mobility management network device according to claim 4, wherein there are a plurality of pieces of second virtual interface information; andthe instructions instruct the at least one processor to verify validity of the current service of the residential gateway based on the first virtual interface information and the second virtual interface information comprises:if the first virtual interface information matches any one of the plurality of pieces of second virtual interface information, determine that the current service of the residential gateway is valid.