What are the Spanning-Tree protocols that are considered open standards?

The Spanning-Tree protocols that are considered open standards include those outlined in the IEEE specifications. The protocol identified as 802.1D is the original standard for the Spanning Tree Protocol (STP), which defines the methodology for preventing loops in Ethernet networks. STP enables a network to maintain a loop-free topology by selectively blocking redundant paths. Being under the IEEE's standards, it is recognized and used broadly across multiple vendors and network devices.

In contrast, while PVRSTP+ and GLBP are relevant to network operations, they are not classified as open standards set by the IEEE. PVRSTP and similar variations are proprietary implementations often specific to certain vendors. Similarly, GLBP (Gateway Load Balancing Protocol) is a Cisco proprietary protocol used for load balancing among gateways, which means it is not an open standard since it’s exclusive to Cisco technology.

802.1W, which refers to Rapid Spanning Tree Protocol (RSTP), is also an open standard, but in the context of the specific question regarding which protocols are recognized as open standards for spanning-tree, 802.1D stands clearly as the foundational reference point.