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In Cisco SD-WAN, once a data tunnel is established, bi-directional forwarding detection (BFD) is automatically started on top and cannot be disabled. It runs on all data tunnels across all WAN transports. That is because multiple advanced features depend on BFD. Edge routers use BFD to detect tunnels' link-state and measure real-time performance characteristics such as latency, jitter, and packet loss. Application-aware Routing policies depend on BFD to provide real-time performance characteristics of every tunnel.

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