STP - UDLD and Fiber Links
A fiber connection using an SFP transceiver typically employs two fiber strands: one for transmission (Tx) and one for reception (Rx). This setup can lead to a unidirectional link if one of the strands fails. Historically, the Tx and Rx operations were independent, meaning one strand could fail while the other continued to function, potentially causing Layer 2 (L2) loops.
Modern transceivers often include mechanisms to detect such failures and will consider the link down if either strand fails. However, these mechanisms may take several seconds to activate, during which the link might still appear operational despite a failure.
To mitigate these risks, especially in high-speed, mission-critical core networks, protocols like LoopGuard and Unidirectional Link Detection (UDLD) are essential. These protocols help ensure network stability by detecting and preventing unidirectional link failures.
Links:
https://forum.networklessons.com/t/spanning-tree-loopguard-and-udld/1145/65?u=lagapidis
https://networklessons.com/spanning-tree/spanning-tree-loopguard-udld