Dell Force10 S4820T S4820T Configuration Guide - Page 956

www.dell.com | support.dell.com 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 View only the root information using the show spanning-tree root command (see Figure 48-5) from EXEC privilege mode. Figure 48-5. 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 STP Root Guard is supported only on platforms: c e s S4820T 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 used 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 48-6 (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. The flow of STP BPDUs is shown in the illustration. In STP topology 2 (Figure 48-6 upper right), STP is enabled on device D on which a software bridge application is started to connect to the network. Because the priority of the bridge in device D is lower than the root bridge in Switch A, device D is elected as root, causing the link between Switches A and B to enter a blocking state. Network traffic then begins to flow in the directions indicated by the BPDU arrows in the topology. If the links between Switches C and A or Switches C and B cannot handle the increased traffic flow, frames may be dropped. 956 | Spanning Tree Protocol (STP)