Dell Force10 MXL Blade MXL 10/40GbE Switch IO Module Configuration Guide - Page 591

STP Root Selection STP determines the root bridge, but you can assign one bridge a lower priority to increase the likelihood that it is selected as the root bridge. You can also specify that a bridge is the root or the secondary root. To change the bridge priority or specify that a bridge is the root or secondary root, use the following command: Task Command Syntax Assign a number as the bridge priority or designate it as the root or secondary root. priority-value range: 0 to 65535. The lower the number assigned, the more likely this bridge will become the root bridge. 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} Command Mode PROTOCOL SPANNING TREE To view only the root information, use the show spanning-tree root command from EXEC privilege mode (Figure 33-11). Figure 33-11. show spanning-tree root Command Example FTOS#show spanning-tree 0 root Root ID Priority 32768, Address 0001.e80d.2462 We are the root of the spanning tree Root Bridge hello time 2, max age 20, forward delay 15 FTOS# STP Root Guard Use the STP root guard feature in a Layer 2 network to avoid bridging loops. In STP, the switch in the network with the lowest priority (as determined by STP or set with the bridge-priority command) is selected as the root bridge. If two switches have the same priority, the switch with the lower MAC address is selected as the root. All other switches in the network use the root bridge as the reference to calculate the shortest forwarding path. Because any switch in an STP network with a lower priority can become the root bridge, the forwarding topology may not be stable. The location of the root bridge can change, resulting in unpredictable network behavior. The STP root guard feature ensures that the position of the root bridge does not change. Root Guard Scenario For example, in Figure 33-12 (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 and Switch B is in a Blocking state. Figure 33-12 shows the flow of STP BPDUs. Spanning Tree Protocol (STP) | 589