3Com 3C17300A Implementation Guide - Page 43

How STP Works 43 How STP Works When enabled, STP determines the most appropriate path for traffic through a network. It does this as outlined in the sections below. STP Requirements Before it can configure the network, the STP system requires: ■ Communication between all the bridges. This communication is carried out using Bridge Protocol Data Units (BPDUs), which are transmitted in packets with a known multicast address. ■ Each bridge to have a Bridge Identifier. This specifies which bridge acts as the central reference point, or Root Bridge, for the STP system - the lower the Bridge Identifier, the more likely the bridge is to become the Root Bridge. The Bridge Identifier is calculated using the MAC address of the bridge and a priority defined for the bridge. The default priority of your Switch is 32768. ■ Each port to have a cost. This specifies the efficiency of each link, usually determined by the bandwidth of the link - the higher the cost, the less efficient the link. Table 3 shows the default port costs for a Switch. Table 3 Default port costs Port Speed Link Type Path Cost 802.1D-1998 Path Cost 802.1w 10 Mbps Half Duplex 100 Full Duplex 95 Aggregated Link 90 2,000,000 1,999,999 1,000,000* 100 Mbps Half Duplex 19 Full Duplex 18 Aggregated Link 15 1000 Mbps Full Duplex 4 Aggregated Link 3 200,000 199,999 100,000* 20,000 10,000* * This path cost is correct where there are two ports in an aggregated link. However, if there are more ports in the aggregated link, the path cost will be proportionately lower. For example, if there are four ports in the aggregated link, the 802.1w path costs will be: 500,000 for 10 Mbps, 50,000 for 100 Mbps, and 5,000 for 1000 Mbps. The 802.1D-1998 path cost values are not affected by the number of ports in an aggregated link. STP Calculation The first stage in the STP process is the calculation stage. During this stage, each bridge on the network transmits BPDUs that allow the system to work out: