Loop Protection

4.1 Context
 

On the network shown in the preceding figure, SW1 is the root bridge. GE0/0/23 on SW3 is a non-designated port and listens to BPDUs sent from SW2 though the port in Discarding state.
 

GE0/0/23 on SW3 cannot receive BPDUs from SW2 due to a unidirectional link failure between SW2 and SW3. In this case, SW3 considers that SW2 is faulty and performs STP recalculation. According to STP recalculation, GE0/0/23 becomes a designated port and transitions to the Forwarding state. In this case, a loop occurs since packets can be sent from SW3 to SW2.

4.2 Basic Concepts

Normally, the root port and the port in Discarding state receive BPDUs. When the ports fail to receive BPDUs due to a unidirectional link failure, STP recalculation is performed. The root port becomes a designated port and the port in Discarding state transitions to the Forwarding state. As a result, a loop occurs.
After loop protection is enabled on the root port, if the role of the root port changes, for example, the root port becomes a designated port, the port is blocked, that is, the port transitions to the Discarding state until it receives new BPDUs.
After loop protection is enabled on a blocked port, the port remains in Discarding state even if it is elected as a designated port.
In MSTP, this function can be configured only on the root port, alternate port, and backup port.

4.3 Configuration Example 1 (Enable Loop Protection on the Root Port)
 

·         Without loop protection enabled

 
After STP converges, GE0/0/22 on SW3 is elected as the root port and GE0/0/23 is elected as a designated port in Discarding state.
 

Disable STP on GE0/0/22 of SW1 to simulate a unidirectional link failure. As a result, SW3 cannot receive BPDUs from GE0/0/22 of SW1.
 

After STP converges again, GE0/0/23 of SW3 becomes the root port and GE0/0/22 becomes a designated port. In this case, a loop occurs.

·         With loop protection enabled 

Enable loop protection on GE0/0/22 of SW3.
 

SW3 performs STP recalculation. GE0/0/23 becomes the root port, and GE0/0/22 becomes a designated port. Since GE0/0/22 is enabled with loop protection, the port remains in Discarding state even though it is a designated port. This situation prevents a loop.

4.4 Configuration Example 2 (Enable Loop Protection on a Designated Port)
 

This example enables loop protection on a designated port, as shown in the preceding figure.
 

If a unidirectional link failure occurs between SW2 and SW3, SW3 cannot receive BPDUs from GE0/0/23. Therefore, SW3 performs STP recalculation. According to STP recalculation, GE0/0/23 becomes a designated port but remains in Discarding state since it is enabled with loop protection. This prevents a loop.

To learn more:

Enhancing STP and Configuring STP Protection Functions part 1

Enhancing STP and Configuring STP Protection Functions part 2