When connecting a switch running classic STP to a switch running RSTP, what mode does the classic STP switch operate in?

When a switch running classic Spanning Tree Protocol (STP) connects to a switch that employs Rapid Spanning Tree Protocol (RSTP), the classic STP switch operates in classic STP mode. This is because the classic STP and RSTP protocols have different mechanisms for port roles and state changes.

Classic STP adheres to the original 802.1D standard, which defines port states such as Blocking, Listening, Learning, and Forwarding. It operates relatively slowly, with longer convergence times when topology changes occur.

On the other hand, RSTP, defined in 802.1w, introduces faster convergence through new port states and the immediate transition of ports to Forwarding state. However, when a classic STP switch is part of a network that includes RSTP switches, it cannot adopt the faster mechanisms of RSTP; it instead continues to use its original STP mechanisms.

The correct choice highlights that even in an environment where RSTP operates, the classic STP switch retains its standard operation without adapting to RSTP features or enhancements. Understanding this distinction is critical to grasping how different spanning tree protocols intermingle and impact network performance.