STP Loop Guard

Fiber-optic cables consist of strands of glass/plastic that transmit light. A cable typically consists of one strand for sending data and another strand for receiving data on one side; the order is directly opposite at the remote site. Network devices that use fiber for connectivity can encounter unidirectional traffic flows if one strand is broken.

In such scenarios, the interface still shows a line-protocol up state; however, BPDUs are not able to be transmitted, and the downstream switch eventually times out the existing root port and identifies a different port as the root port. Traffic is then received on the new root port and forwarded out the strand that is still working, thereby creating a forwarding loop.

STP loop guard prevents any alternative or root ports from becoming designated ports (ports toward downstream switches) due to loss of BPDUs on the root port. Loop guard places the original port in an ErrDisabled state while BPDUs are not being received. When BPDU transmission starts again on that interface, the port recovers and begins to transition through the STP states again.

For additional STP fine tuning features, take a look at STP fine tuning the spanning tree protocol.

Links

https://networklessons.com/spanning-tree/spanning-tree-loopguard-udld

https://www.ciscopress.com/articles/article.asp?p=2995351&seqNum=3