Spanning Tree Protocol 5-15An ExampleFigure 5-13 illustrates part of a network. All bridgeshave a path cost value assigned to each port, identi-fied by PC=xxx (where xxx is the value).Bridge A is selected by STP as the Root Bridge,because it has the lowest Bridge Identifier. The Des-ignated Bridge Port for LAN A is port 1 on Bridge A.Each of the other four bridges have a Root Port (theport closest to the Root Bridge). Bridge X and BridgeB can offer the same path cost to LAN B. In thiscase Bridge B's port is chosen as the DesignatedBridge Port, because it has the lowest Bridge Identi-fier. Bridge C's port is chosen as the DesignatedBridge Port for LAN C because it offers the lowestRoot Path Cost (the route through Bridge C and Bcosts 200, the route through Bridge Y and B wouldcost 300). You can set the path cost of a bridgeport to influence the configuration of a networkwith a duplicate path.Once the network topology is stable, all the bridgeslisten for special Hello BPDUs transmitted from theRoot Bridge at regular intervals. If the STP Max Agetime expires (refer to “Configuring the STP Parame-ters of VLANs” on page 5-18) before receiving aHello BPDU, it assumes that the Root Bridge, or alink between itself and the Root Bridge, has gonedown. It then initiates a reconfiguration of the net-work topology.You can adjust timers to determine how quickly anetwork reconfigures and therefore how rapidly thenetwork recovers from a path failure (refer to “Con-figuring the STP Parameters of VLANs” on page5-18). Figure 5-13 Port costs in a network
