Cisco 2950 Software Configuration Guide - Page 324

Understanding RSTP Chapter 15 Configuring MSTP Understanding RSTP The 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), which is critical for networks carrying delay-sensitive traffic such as voice and video. These section describes how the RSTP works: • Port Roles and the Active Topology, page 15-6 • Rapid Convergence, page 15-7 • Synchronization of Port Roles, page 15-8 • Bridge Protocol Data Unit Format and Processing, page 15-9 For configuration information, see the "Configuring MSTP Features" section on page 15-11. Port Roles and the Active Topology The RSTP provides rapid convergence of the spanning tree by assigning port roles and by determining 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 14-3. 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 together 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 15-1 provides a comparison of 802.1D and RSTP port states. 15-6 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 78-11380-10