In RSTP, how many BPDUs must a switch miss before it considers a neighbor lost?

In Rapid Spanning Tree Protocol (RSTP), a switch determines the status of its neighboring switches based on the receipt of Bridge Protocol Data Units (BPDUs). For a switch to conclude that a neighbor is lost, it must miss receiving BPDUs from that neighbor for a specified number of intervals.

RSTP uses a mechanism called the "Max Age" timer, which is typically set to 20 seconds by default. After a switch misses a certain number of BPDUs from a neighbor—specifically, three—it will consider that neighbor to be down or lost. This approach allows for a quick detection of failures in the network and enables the protocol to converge rapidly by updating its topology.

Therefore, the answer is based on the operational behavior of RSTP concerning the resilience and fault tolerance of switch-to-switch communication within the network. The process ensures that changes in network topology are recognized and acted upon promptly, which is crucial for maintaining an efficient and loop-free switching environment.