How STP Works 51 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 4 shows the default port costs fora Switch.Table 4 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. The identity of the port on each bridge that is to be the Root Port.The Root Port is the one that is connected to the Root Bridge usingthe most efficient path, that is, the one that has the lowest RootPath Cost. Note that the Root Bridge does not have a Root Port. The identity of the bridge that is to be the Designated Bridge ofeach LAN segment. The Designated Bridge is the one that has thelowest Root Path Cost from that segment. Note that if severalbridges have the same Root Path Cost, the one with the lowestBridge Identifier becomes the Designated Bridge.Port Speed Link TypePath Cost 802.1D,1998 EditionPath Cost802.1w-200110 Mbps Half DuplexFull DuplexAggregated Link10095902,000,0001,999,9991,000,000 ** This path cost is correct where there are two ports in an aggregated link. However, if there aremore ports in the aggregated link, the path cost will be proportionately lower. For example, ifthere are four ports in the aggregated link, the 802.1w path costs will be: 500,000 for10 Mbps, 50,000 for 100 Mbps, and 5,000 for 1000 Mbps. The 802.1D, 1998 Edition pathcost values are not affected by the number of ports in an aggregated link.100 Mbps Half DuplexFull DuplexAggregated Link191815200,000199,999100,000*1000 Mbps Full DuplexAggregated Link4320,00010,000*
