Cisco WS-C2960-24TC-S Software Guide - Page 333

Chapter 16 Configuring MSTP Understanding RSTP These sections describe how the RSTP works: • Port Roles and the Active Topology, page 16-9 • Rapid Convergence, page 16-10 • Synchronization of Port Roles, page 16-11 • Bridge Protocol Data Unit Format and Processing, page 16-12 For configuration information, see the "Configuring MSTP Features" section on page 16-14. 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 IEEE 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value) as the root switch as described in the "Spanning-Tree Topology and BPDUs" section on page 15-3. Then the RSTP 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 switch. • Designated port-Connects to the designated switch, which incurs the lowest path cost when forwarding packets from that LAN to the root switch. 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 switch 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 IEEE 802.1D). The port state controls the operation of the forwarding and learning processes. Table 16-2 provides a comparison of IEEE 802.1D and RSTP port states. Table 16-2 Port State Comparison Operational Status Enabled Enabled Enabled Enabled Disabled STP Port State (IEEE 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 defines the port state as blocking instead of discarding. Designated ports start in the listening state. OL-8603-04 Catalyst 2960 Switch Software Configuration Guide 16-9