A network designer is redesigning an enterprise campus network to ensure that Ethernet switches proactively attempt to reconnect after a fiber cut. In the design, they will have to address areas where

Loop guard protects ports from transitioning to a forwarding state due to missing BPDUs on point-to-point links but has no mechanism to detect or react to the underlying unidirectional physical link condition caused by a broken fiber strand.

UDLD normal mode detects unidirectional links and logs a syslog alert but does not err-disable the port, leaving the potentially loop-inducing link active until an administrator takes manual corrective action.

UDLD aggressive mode sends periodic echo messages on fiber links and, when a neighbor fails to respond within the defined interval (indicating a unidirectional link), immediately err-disables the port rather than only generating a syslog alert. This proactive shutdown prevents STP from continuing to forward over a link that carries traffic in only one direction, which would otherwise produce bridging loops that normal STP cannot detect.

Root guard enforces STP root bridge placement by blocking ports that receive superior BPDUs from unauthorized switches and has no capability to detect or respond to unidirectional fiber link failures.