Cisco 3560G 48TS Software Configuration Guide - Page 330

Understanding RSTP Chapter 16 Configuring MSTP When Switch C is connected to Switch B, a similar set of handshaking messages are exchanged. Switch C selects the port connected to Switch B as its root port, and both ends immediately transition to the forwarding state. With each iteration of this handshaking process, one more switch joins the active topology. As the network converges, this proposal-agreement handshaking progresses from the root toward the leaves of the spanning tree. The switch learns the link type from the port duplex mode: a full-duplex port is considered to have a point-to-point connection; a half-duplex port is considered to have a shared connection. You can override the default setting that is controlled by the duplex setting by using the spanning-tree link-type interface configuration command. Figure 16-2 Proposal and Agreement Handshaking for Rapid Convergence Switch A 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 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 16-3. 16-8 Catalyst 3560 Switch Software Configuration Guide 78-16156-01