Cisco 3845 Software Guide - Page 297

Chapter 14 Configuring MSTP Understanding RSTP Figure 14-4 Switch A Proposal and Agreement Handshaking for Rapid Convergence Proposal Switch B Root F Agreement Designated switch F DP RP Root F DP Designated switch F RP Proposal Switch C Root F DP Designated switch F F RP DP Agreement F RP DP = designated port RP = root port F = forwarding 88760 Synchronization of Port Roles When the switch receives a proposal message on one of its ports and that port is selected as the new root port, the RSTP forces all other ports to synchronize with the new root information. The switch is synchronized with superior root information received on the root port if all other ports are synchronized. An individual port on the switch is synchronized if • That port is in the blocking state. • It is an edge port (a port configured to be at the edge of the network). If a designated STP port is in the forwarding state and is not configured as an edge port, it transitions to the blocking state when the RSTP forces it to synchronize with new root information. In general, when the RSTP forces a port to synchronize with root information and the port does not satisfy any of the above conditions, its port state is set to blocking. After ensuring all of the ports are synchronized, the switch sends an agreement message to the designated switch corresponding to its root port. When the switches connected by a point-to-point link are in agreement about their port roles, the RSTP immediately transitions the port states to forwarding. The sequence of events is shown in Figure 14-5. OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 14-11