Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.1P1 - Page 902

In STP topology 3 (shown in the lower middle), if you have enabled the root guard feature 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). As a result, Switch A becomes the root bridge. Figure 126. STP Root Guard Prevents Bridging Loops Configuring Root Guard Enable STP root guard on a per-port or per-port-channel basis. Dell Networking OS Behavior: The following conditions apply to a port enabled with STP root guard: • Root guard is supported on any STP-enabled port or port-channel interface except when used as a stacking port. • Root guard is supported on a port in any Spanning Tree mode: • Spanning Tree Protocol (STP) • Rapid Spanning Tree Protocol (RSTP) • Multiple Spanning Tree Protocol (MSTP) • Per-VLAN Spanning Tree Plus (PVST+) • When enabled on a port, root guard applies to all VLANs configured on the port. • You cannot enable root guard and loop guard at the same time on an STP port. For example, if you configure root guard on a port on which loop guard is already configured, the following error message displays: • % Error: LoopGuard is configured. Cannot configure RootGuard. • When used in an MSTP network, if root guard blocks a boundary port in the CIST, the port is also blocked in all other MST instances. To enable the root guard on an STP-enabled port or port-channel interface in instance 0, use the following command. 902 Spanning Tree Protocol (STP)