Bidirectional Forwarding Detection (BFD) is a network protocol designed for rapid detection of path failures. When used with BGP over a Direct Connect connection, BFD can detect a failure in the path in under a second, compared to the default BGP hold-down timer which can take 90 seconds or more. This allows for near-instantaneous failover to the secondary connection. According to AWS documentation, asynchronous BFD is automatically enabled on the AWS side of a Direct Connect virtual interface when it is configured on the customer's on-premises router. Therefore, configuring BFD on the on-premises router is the direct action that implements this high-speed failover mechanism, providing the largest reduction in failover time.

A. Reducing the BGP hold-down timer improves failover time but is still measured in multiple seconds and is significantly slower than BFD's sub-second detection.

B. A CloudWatch alarm and Lambda function approach is reactive and introduces significant latency from detection, alarm state change, and function invocation, resulting in a failover time of minutes.

C. While BFD must be active on the AWS side, this is enabled automatically in response to the on-premises router configuration. Therefore, this option is less precise than D.

1. AWS Direct Connect User Guide, "Bidirectional Forwarding Detection (BFD)" section: "BFD liveness detection provides a faster detection of path failures than BGP timers... Asynchronous BFD is automatically enabled for Direct Connect virtual interfaces on the AWS side when you enable it on your router." This confirms that BFD is the fastest method and the primary configuration step is on the on-premises equipment.

2. AWS Direct Connect User Guide, "Bidirectional Forwarding Detection (BFD)" section: "We recommend that you configure the BFD liveness detection minimum interval to 300, and the BFD liveness detection multiplier to 3." This configuration results in a detection time of less than one second (300ms 3 = 900ms), a significant reduction from the default BGP hold timer of 90 seconds.

3. AWS Blog, "AWS Direct Connect introduces Bidirectional Forwarding Detection (BFD) for faster failover" (Nov 19, 2019): "With BFD, path failure detection time can be reduced to less than a second, compared to the default BGP convergence time of three minutes, or 180 seconds." This highlights the magnitude of the failover time reduction provided by BFD.