Cisco 7609-S Software Configuration Guide - Page 287

Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST Understanding How IEEE 802.1w RSTP Works For more information on 802.1Q trunks, see Chapter 10, "Configuring LAN Ports for Layer 2 Switching." Understanding How IEEE 802.1w RSTP Works Note RSTP is available as a standalone protocol in Rapid-Per-VLAN-Spanning Tree (Rapid-PVST) mode. In this mode, the router runs an RSTP instance on each VLAN, which follows the usual PVST+ approach. These sections describe Rapid Spanning Tree Protocol (RSTP): • IEEE 802.1w RSTP Overview, page 20-13 • RSTP Port Roles, page 20-13 • RSTP Port States, page 20-14 • Rapid-PVST, page 20-14 IEEE 802.1w RSTP Overview RSTP significantly reduces the time to reconfigure the active topology of the network when changes occur to the physical topology or its configuration parameters. RSTP selects one router as the root of a spanning tree-connected active topology and assigns port roles to individual ports of the router, depending on whether that port is part of the active topology. RSTP provides rapid connectivity following the failure of a router, router port, or a LAN. A new root port and the designated port on the other side of the bridge transition to forwarding using an explicit handshake between them. RSTP allows router port configuration so that the ports can transition to forwarding directly when the router reinitializes. RSTP as specified in 802.1w supersedes STP specified in 802.1D, but remains compatible with STP. RSTP provides backward compatibility with 802.1D bridges as follows: • RSTP selectively sends 802.1D-configured BPDUs and topology change notification (TCN) BPDUs on a per-port basis. • When a port initializes, the migration-delay timer starts and RSTP BPDUs are transmitted. While the migration-delay timer is active, the bridge processes all BPDUs received on that port. • If the bridge receives an 802.1D BPDU after a port's migration-delay timer expires, the bridge assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs. • When RSTP uses 802.1D BPDUs on a port and receives an RSTP BPDU after the migration-delay expires, RSTP restarts the migration-delay timer and begins using RSTP BPDUs on that port. RSTP Port Roles RSTP uses the following definitions for port roles: • Root-A forwarding port elected for the spanning tree topology. • Designated-A forwarding port elected for every switched LAN segment. • Alternate-An alternate path to the root bridge to that provided by the current root port. OL-4266-08 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX 20-13