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There is no autonomous way for a WAN edge router to understand which interface is connected to which WAN network. The concept of WAN clouds, such as the Internet, MPLS, LTE, and Metro Ethernet, makes sense only to humans. From the router’s point of view, WAN uplinks look the same. They have IP addresses, and they learn the MAC address of the next-hop layer 3 device. How can a router understand which remote devices are reachable through a specific WAN interface? 

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