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[All About Switches] Layer 2 Loop Troubleshooting Highlighted

Latest reply: Oct 26, 2016 03:34:55 4006 1 0 0 1

Hello everyone,

Today I will share with you some information about Layer 2 Loop Troubleshooting.


To improve the reliability of an Ethernet switching network, device redundancy and link redundancy are commonly used methods. However, many factors such as networking adjustment, configuration modification, and upgrade/migration, loops may still occur. In Figure 1-1, loops will occur if every two devices are connected, and the broadcast storm will occur if no loop prevention protocol is configured or network configurations are modified.

Figure 1-1 Link redundancy on the Ethernet switching network

The major harm of the Layer 2 loop is a broadcast storm. If no loop has occurred on an Ethernet, broadcast Ethernet frames are flooded on the network to ensure that they can be received by every device. With sufficient bandwidth, each bridge forwards received broadcast frames to all interfaces except the interface receiving these frames. However, if a loop occurs, this broadcast mechanism will affect the entire network.

When a broadcast storm is generated, Ethernet frames are forwarded permanently, and the forwarding speed reaches or approximates the line speed on an interface to consume link bandwidth. According to Ethernet forwarding rules, these broadcast frames are copied to all interfaces. Therefore, the entire network is full of broadcast frames. Assume that an Ethernet uses GE connections, every link is full of broadcast frames at the speed of 1000 M/s. Other data packets cannot be forwarded.

In a broadcast domain, if Layer 2 devices forward broadcast frames repeatedly, a broadcast storm will occur. The broadcast storm causes the MAC table unstable, affects services, degrades communication quality, or even interrupts communication.

To prevent loops and ensure network reliability, the following loop prevention protocols can be configured on switches:


  • RRPP

  • SEP

  • Smart Link

  • ERPS

In addition, Huawei S series switches support the following loop detection functions:

  • Loop Detection

  • Loopback Detection

This document describes how to identify Layer 2 loops.


This is a guide for Huawei engineers to remove Layer 2 loops, including:

  • Helping frontline service engineers describe the fault symptom and determine fault range

  • Helping GTAC engineers collect NE information, analyze abnormalities of NEs, and quickly locate the faulty NE and service

  • Helping R&D engineers locate the fault

★★★Summary★★★ All About Huawei Switch Features and Configurations

That is all I want to share with you! Thank you!

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Created Oct 26, 2016 03:34:55

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