When Bidirectional Forwarding Detection (BFD) detects a fault, it changes the interface status in the port state table (PST) to trigger a fast reroute (FRR) switchover. BFD for PST applies only to single-hop scenarios when BFD sessions are bound to outbound interfaces.
BFD for PST is widely used in FRR applications. If BFD for PST is enabled for a BFD session bound to an outbound interface, the BFD session is associated with the PST on the outbound interface. After BFD detects that a link is Down, it sets the bit for the PST to Down to immediately trigger an FRR switchover.