D-Link DGS-6600-48T Configuration Guide - Page 132

Volume 3-Layer 2 Configurations / Chapter 14-Spanning Tree Protocol (STP) Configuration Chapter Overview 4) Suppose that a port on bridge Y receives a BPDU from bridge X, the priority vector of the message will be calculated based on the priority conveyed by the received BPDU. 5) (Bridge ID-X, RPC, Bridge ID-X, Bridge ID-X TX port, Bridge ID-Y RX Port) 6) The message priority vector will be compared against the designated priority vector of the RX port, with the best one becoming the port priority vector. If the designated priority vector is better, the port's role will be designated port. If the message priority vector is better, the port's role will be either root port or alternate port. The root path priority vector of the root port or alternate port is calculated by adding the link path cost to the RPC. 7) (Bridge ID-X, RPC+ RX port Path Cost, Bridge ID-X, Bridge ID-X TX port, Bridge ID-Y RX Port) 8) Based on the new derived root path priority vector, the bridge will re-calculate the new root priority vector if necessary. 9) (Bridge ID-X, RPC+ RX port Path Cost, Bridge ID-X, Bridge ID-X TX port, Bridge ID-Y RX Port) 10) Based on the new root priority vector, the bridge re-calculates each port's designated priority vector. This recalculation may cause the re-evaluation of Step-4 and port role re-assignment. 11) (Bridge ID-X, RPC+ RX port Path Cost, Bridge ID-X, TX port, TX Port) Rapid Spanning Tree Protocol (RSTP) Concepts Rapid Spanning Tree Protocol is a protocol defined by IEEE 802.1w, which improves the convergence time of 802.1d. In 802.1d, when the topology changes and a port is re-computed to an active state, the port will take twice the amount of time specified by the forward delay time to change from the blocking state to the forwarding state. 802.1w reduces this convergence time by applying a handshake algorithm between ports on the neighbor switches and ports on the same switch so that a port can change from the blocking state to the forwarding state within the forward delay time. In addition to modifying the protocol's operation, the definition of the Bridge ID format and Port ID format have also changed. Change of Bridge ID Format The Bridge ID in 802.1d is formed by a 16-bit bridge priority and 6 bytes of the MAC Address. In 802.1w, the 16-bit bridge priority is divided into a 4-bit port priority and a 12-bit Extension System ID. In 802.1w the Extension System ID part is blank and in 802.1s the Extension System ID is the same as the VLAN ID. Change of Port ID Format The Port ID in 802.1d is formed by an 8-bit priority and 8-bit port number. In 802.1w, the Port ID is formed by a 4-bit port priority and a 12-bit port number. Protocol Migration In the network topology, some network nodes may run 802.1d and some switches may run 802.1w. Bridge nodes running 802.1w are backward compatible with 802.1d in the following way. Suppose that on an 802.1w node some ports are connected to 802.1w nodes and some ports are connected to 802.1d nodes. 802.1w will be able to automatically detect the version and downgrade to the 802.1d version for ports that are attached to 802.1d nodes. DGS-6600 Configuration Guide 132