Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 236

Understanding How IEEE 802.1w RSTP Works Chapter 17 Configuring STP and MST Understanding How IEEE 802.1w RSTP Works RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree. Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings in the 802.1D spanning tree). These section describes how the RSTP works: • Port Roles and the Active Topology, page 17-12 • Rapid Convergence, page 17-13 • Synchronization of Port Roles, page 17-14 • Bridge Protocol Data Unit Format and Processing, page 17-15 • Topology Changes, page 17-17 • Rapid-PVST, page 17-17 Port Roles and the Active Topology The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the active topology. The RSTP builds upon the 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value) as the root bridge as described in the "Election of the Root Bridge" section on page 17-4. The RSTP then assigns one of these port roles to individual ports: • Root port-Provides the best path (lowest cost) when the switch forwards packets to the root bridge. • Designated port-Connects to the designated switch, which incurs the lowest path cost when forwarding packets from that LAN to the root bridge. The port through which the designated switch is attached to the LAN is called the designated port. • Alternate port-Offers an alternate path toward the root bridge to that provided by the current root port. • Backup port-Acts as a backup for the path provided by a designated port toward the leaves of the spanning tree. A backup port can exist only when two ports are connected in a loopback by a point-to-point link or when a switch has two or more connections to a shared LAN segment. • Disabled port-Has no role within the operation of the spanning tree. A port with the root or a designated port role is included in the active topology. A port with the alternate or backup port role is excluded from the active topology. 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 17-3 provides a comparison of 802.1D and RSTP port states. Table 17-3 Port State Comparison Operational Status Enabled Enabled Enabled STP Port State (IEEE 802.1D) Blocking Listening Learning RSTP Port State Discarding Discarding Learning Is Port Included in the Active Topology? No No Yes 17-12 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03