3Com SuperStack 3 3812 Implementation Manual

Contents

How STP Works 43How STP Works When enabled, STP determines the most appropriate path for trafficthrough 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 iscarried out using Bridge Protocol Data Units (BPDUs), which aretransmitted in packets with a known multicast address.■ Each bridge to have a Bridge Identifier. This specifies which bridge actsas the central reference point, or Root Bridge, for the STP system —the lower the Bridge Identifier, the more likely the bridge is to becomethe Root Bridge. The Bridge Identifier is calculated using the MACaddress of the bridge and a priority defined for the bridge. The defaultpriority 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 thecost, the less efficient the link. Table 3 shows the default port costs fora Switch.Table 3 Default port costsSTP Calculation The first stage in the STP process is the calculation stage. During thisstage, each bridge on the network transmits BPDUs that allow the systemto work out:■ The identity of the bridge that is to be the Root Bridge. The RootBridge is the central reference point from which the network isconfigured.■ The Root Path Costs for each bridge — that is, the cost of the pathsfrom each bridge to the Root Bridge.Port Speed Link TypePath Cost 802.1D,1998 EditionPath Cost802.1w-200110 Mbps Half DuplexFull DuplexAggregated Link10095902,000,0001,999,9991,000,000100 Mbps Half DuplexFull DuplexAggregated Link191815200,000199,999100,0001000 Mbps Full DuplexAggregated Link4320,00010,000