As shown in the following illustration (STP topology 2, upper right), a loop can also be created if the forwarding port on Switch Bbecomes busy and does not forward BPDUs within the configured forward-delay time. As a result, the blocking port on SwitchC transitions to a forwarding state, and both Switch A and Switch C transmit traffic to Switch B (STP topology 2, lower right).As shown in STP topology 3 (bottom middle), after you enable loop guard on an STP port or port-channel on Switch C, if no BPDUsare received and the max-age timer expires, the port transitions from a blocked state to a Loop-Inconsistent state (instead of to aForwarding state). Loop guard blocks the STP port so that no traffic is transmitted and no loop is created.As soon as a BPDU is received on an STP port in a Loop-Inconsistent state, the port returns to a blocking state. If you disable STPloop guard on a port in a Loop-Inconsistent state, the port transitions to an STP blocking state and restarts the max-age timer.Figure 110. STP Loop Guard Prevents Forwarding LoopsConfiguring Loop GuardEnable STP loop guard on a per-port or per-port channel basis.Dell Networking OS Behavior: The following conditions apply to a port enabled with loop guard:• Loop guard is supported on any STP-enabled port or port-channel interface.• Loop guard is supported on a port or port-channel in any spanning tree mode:722 Spanning Tree Protocol (STP)
