Dell Force10 E600i FTOS Version 8.4.1.5 Configuration Guide for ExaScale - Page 992

www.dell.com | support.dell.com View only the root information using the show spanning-tree root command (refer to the following example) from EXEC privilege mode. 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 c e s STP Root Guard is supported only on platforms: t 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 55-4 (topology on far 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 the second topology (far right in Figure 55-4), 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. If the root guard feature is enabled on the STP port on Switch C that connects to device D, and device D sends a superior BPDU that would trigger the election of device D as the new root bridge, the BPDU is ignored and the port on Switch C transitions from a forwarding to a root-inconsistent state (shown by the green X icon in the lower topology in the middle of Figure 55-4). Switch A becomes the root bridge. All incoming and outgoing traffic is blocked on an STP port in a root-inconsistent state. After the timeout period, the Switch C port automatically transitions to a forwarding state as soon as device D stops sending BPDUs that advertise a lower priority. If you enable a root guard on all STP ports on the links where the root bridge should not appear, you can ensure a stable STP network topology and avoid bridging loops. 992 | Spanning Tree Protocol