Dell EMC Networking OS Configuration Guide for the S5048F–ON System 9.14.2.4

The STP loop guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by a hardware failure, such as a cable failure or an interface fault. When a cable or interface fails, a participating STP link may become unidirectional (STP requires links to be bidirectional) and an STP port does not receive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to a Forwarding state. This condition can create a loop in the network.

For example, in the following example (STP topology 1, upper left), Switch A is the root switch and Switch B normally transmits BPDUs to Switch C. The link between Switch C and Switch B is in a Blocking state. However, if there is a unidirectional link failure (STP topology 1, lower left), Switch C does not receive BPDUs from Switch B. When the max-age timer expires, the STP port on Switch C becomes unblocked and transitions to Forwarding state. A loop is created as both Switch A and Switch C transmit traffic to Switch B.

As shown in the following illustration (STP topology 2, upper right), a loop can also be created if the forwarding port on Switch B becomes busy and does not forward BPDUs within the configured forward-delay time. As a result, the blocking port on Switch C 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 BPDUs are received and the max-age timer expires, the port transitions from a blocked state to a Loop-Inconsistent state (instead of to a Forwarding 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 STP loop guard on a port in a Loop-Inconsistent state, the port transitions to an STP blocking state and restarts the max-age timer.