Spanning Tree Protocol (STP) convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. When layer 2 is converged, Root Switch is elected and Root Ports, Designated Ports and Non-Designated ports in all switches are selected. At Converged condition, the Root Ports and the Designated ports are in forwarding state, and all other ports are in blocking state.
For Layer 2 switches, convergence occurs once Spanning Tree Protocol (STP) process has completed: a Root Switch is elected, Root Ports and Designated Ports have been chosen, the Root Ports and Designated Ports have been placed in a forwarding state, and all other ports have been placed in a blocking state.
If a port has to go through all four states, convergence takes 50 seconds: 20 seconds in blocking, 15 seconds in listening, and 15 seconds in learning.
If a port doesn’t have to go through the blocking state but starts at a listening state, convergence takes only 30 seconds. This typically occurs when the Root Port is still valid, but another topology change has occurred.
Remember that during this time period (until the port reaches a forwarding state), no user traffic is forwarded through the port.