Cisco 3560G 48TS Software Configuration Guide - Page 329

Chapter 16 Configuring MSTP Understanding RSTP In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state (equivalent to blocking in 802.1D). The port state controls the operation of the forwarding and learning processes. Table 16-1 provides a comparison of 802.1D and RSTP port states. Table 16-1 Port State Comparison Operational Status Enabled Enabled Enabled Enabled Disabled STP Port State (802.1D) Blocking Listening Learning Forwarding Disabled RSTP Port State Discarding Discarding Learning Forwarding Discarding Is Port Included in the Active Topology? No No Yes Yes No To be consistent with Cisco STP implementations, this guide documents the port state as blocking instead of discarding. Designated ports start in the listening state. Rapid Convergence The RSTP provides for rapid recovery of connectivity following the failure of a switch, a switch port, or a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through point-to-point links as follows: • Edge ports-If you configure a port as an edge port on an RSTP switch by using the spanning-tree portfast interface configuration command, the edge port immediately transitions to the forwarding state. An edge port is the same as a Port Fast-enabled port, and you should enable it only on ports that connect to a single end station. • Root ports-If the RSTP selects a new root port, it blocks the old root port and immediately transitions the new root port to the forwarding state. • Point-to-point links-If you connect a port to another port through a point-to-point link and the local port becomes a designated port, it negotiates a rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-free topology. As shown in Figure 16-2, Switch A is connected to Switch B through a point-to-point link, and all of the ports are in the blocking state. Assume that the priority of Switch A is a smaller numerical value than the priority of Switch B. Switch A sends a proposal message (a configuration BPDU with the proposal flag set) to Switch B, proposing itself as the designated switch. After receiving the proposal message, Switch B selects as its new root port the port from which the proposal message was received, forces all nonedge ports to the blocking state, and sends an agreement message (a BPDU with the agreement flag set) through its new root port. After receiving Switch B's agreement message, Switch A also immediately transitions its designated port to the forwarding state. No loops in the network are formed because Switch B blocked all of its nonedge ports and because there is a point-to-point link between Switches A and B. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 16-7