Three-layer virtual private network achieving method and device

A three-layer virtual private network (L3 VPN) achieving method comprises the steps that a first PE receives an access notice message sent by a first terminal device, and the access notice message carries the IP address of the first terminal device and is used for notifying the first PE that the first terminal device is connected to a first site; the first site is connected to an L3 VPN through the first PE; the first PE obtains the IP address of a second PE according to the ID of the VPN associated with an interface receiving the access notice message and the IP address of the first terminal device; a second site is connected to the L3 VPN through the second PE; the first terminal device is connected to the second site before being connected to the first site; the first PE obtains the IP address of a second terminal device according to the IP address of the second PE and the ID of the VPN associated with the interface receiving the access notice message; the second terminal device is connected to the second site; the first PE sends a binding updating notice message to the first terminal device, and the binding updating notice message carries the MAC address of the first PE and the IP address of the second terminal device.