Cisco IE-3000-8TC Software Configuration Guide - Page 399

Chapter 19 Configuring MSTP Understanding RSTP 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 18-3. 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 19-2 provides a comparison of IEEE 802.1D and RSTP port states. Table 19-2 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. 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. OL-13018-03 Cisco IE 3000 Switch Software Configuration Guide 19-9