What is a network loop? How do I find the loop port? How can I prevent it?

This issue shares relevant knowledge about network loops.

1. Loop Introduction

In networking, a "Layer 2 loop" refers to a loop structure that occurs within the second layer (data link layer) of a network. Layer 2 loops can cause broadcast storms, multiple frame duplication, and MAC address table instability, impacting network performance and stability. First, we need to understand the three main types of typical physical loop topologies:

• Type 1: Multiple switches interconnected loop broadcast packets will be wirelessly forwarded between the loop interfaces of multiple switches, causing serious impact on device performance and network.
• Type 2: A loop broadcast packet from the switch's downlink port floods the switch itself and impacts the upstream core devices, paralyzing the entire network.
• Type 3: The composite loop device topology includes both downlink self-loop and interconnection loops between switches.

2. Loop Judgment

A Layer 2 loop in a network usually presents the following symptoms. If you analyze the existing network and find one or more of the following abnormalities, there is a high probability that a Layer 2 loop exists in the network. You can determine whether a Layer 2 loop fault exists by:

3. Find the port

So how do you find the port where the loop occurs? There are two common methods:

Method 1: Physically unplug the cable and shut down the port

See which interface is unplugged or down and then restored to normal. However, this only applies to one ring and cannot be accurately determined in multiple rings.

Method 2: Check the port's packet receiving rate to find the loophole

A port with a sudden and rapid increase in broadcast and multicast packets may have a loop. For switches, we only need to look at the ingress direction (RX).

If there is a sudden increase in broadcast and multicast traffic in the RX direction of only one port, it means that the downlink link of this port has formed a loop (as shown in type 2 in the figure). The switch page is as follows:

If there is a sudden increase in broadcast and multicast traffic in the RX direction of multiple ports, it means that these ports are forming a ring. If there are n abnormal ports, there will be ≥ n/2 rings (as shown in Type 1 and Type 3 in the figure).

4. Destruction Methods 

The existence of Layer 2 loops can have a serious impact on network performance and stability. Therefore, during network design and deployment, effective measures must be taken to prevent and control the occurrence of Layer 2 loops. Common measures include:

• Use Spanning Tree Protocol (STP/RSTP/MSTP);
• Enable loop detection and isolation technology;
• Reasonable network topology design and closing unused ports;

These measures can ensure the normal operation of the network and improve network reliability and performance.