Which protocol is specifically designed to quickly detect network link faults?

BFD, or Bidirectional Forwarding Detection, is a protocol that is specifically designed to rapidly detect link failures in a network. Its primary strength lies in its ability to provide low-overhead, high-speed detection of faults in the forwarding path between two routers or network devices. By sending small, periodic control packets at a configurable interval, BFD can detect failures in the communication link within milliseconds. This quick detection allows for efficient failover mechanisms to be implemented, ensuring minimal disruption to network services.

The other options present protocols that are oriented towards different functionalities. For example, RSTP (Rapid Spanning Tree Protocol) is focused on preventing loops in Ethernet networks and ensuring rapid reconfiguration of the network topology when changes occur. iSPF refers to the Internet Shortest Path First routing protocols used in routing algorithms but does not prioritize link fault detection. Smart-Link is often associated with certain vendor-specific implementations for link aggregation or redundancy but does not have the same dedicated function as BFD in fault detection. Thus, BFD stands out as the protocol expressly designed for quick fault detection in network links.