1060 | Spanning Tree Protocolw w w . d e l l . c o m | s u p p o r t . d e l l . c o m To change the bridge priority or specify that a bridge is the root or secondary root:View only the root information using the show spanning-tree root command (see Figure 52-9) from EXECprivilege mode.Figure 52-9. show spanning-tree root Command ExampleSTP Root GuardSTP Root Guard is supported only on platforms: c e sUse the STP Root Guard feature in a Layer 2 network to avoid bridging loops. In STP, the switch in thenetwork with the lowest priority (as determined by STP or set with the bridge-priority command) is selectedas the root bridge. If two switches have the same priority, the switch with the lower MAC address isselected as the root. All other switches in the network use the root bridge as the reference used to calculatethe shortest forwarding path.Because any switch in an STP network with a lower priority can become the root bridge, the forwardingtopology may not be stable. The location of the root bridge can change, resulting in unpredictable networkbehavior. The STP root guard feature ensures that the position of the root bridge does not change.Root Guard ScenarioFor example, in Figure 52-10 (STP topology 1 upper left) Switch A is the root bridge in the network core.Switch C functions as an access switch connected to an external device. The link between Switch C andSwitch B is in a blocking state. The flow of STP BPDUs is shown in the illustration.Task Command Syntax Command ModeAssign a number as the bridge priority or designate it as theroot or secondary root.priority-value range: 0 to 65535. The lower the numberassigned, the more likely this bridge will become the rootbridge. The default is 32768.• The primary option specifies a bridge priority of 8192.• The secondary option specifies a bridge priority of 16384.bridge-priority {priority-value |primary | secondary}PROTOCOLSPANNING TREEFTOS#show spanning-tree 0 rootRoot ID Priority 32768, Address 0001.e80d.2462We are the root of the spanning treeRoot Bridge hello time 2, max age 20, forward delay 15FTOS#
