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Once all WAN edge routers have joined the overlay, the WAN stops being “a bunch of circuits and box-to-box routing” and becomes a full mesh of IPsec tunnels that are controlled by software (hence software-defined). Under the hood, each site can use any mix of any transports, like MPLS, broadband internet, LTE/5G, or even satellite. The overlay makes them all look like clean, predictable links that are controlled by centralized policies, as shown in Figure 3.17.

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