How to delect Layer 2 Loop of Huawei S5700 Switch?

This acticle will  introduce beriefly how to delect Layer 2 Loop of Huawei S5700 Switch?

Definition

To improve reliability of an Ethernet switching network, device like S5720-28X-SI-AC

redundancy and link redundancy are commonly used. However, many factors such as networking adjustment, configuration modification, and upgrade/migration, may cause protocol or data packets to be forwarded along a loop path. For example, loops will occur if every two devices are connected, as shown in Figure 6-1. Broadcast storm will occur if no loop prevention protocol is configured or network configurations are modified.

Figure 6-1 Link redundancy on the Ethernet switching network

The major harm of a Layer 2 loop is that it causes broadcast storm. If there is no loop 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 receiving interface. However, if a loop occurs, this broadcast mechanism will cause severe faults.

When broadcast storm is generated, Ethernet frames are forwarded permanently, and the forwarding speed reaches or approximates the line speed on an interface, consuming link bandwidth at an enormous speed. According to Ethernet forwarding rules, the devices on the loop will copy these broadcast frames to all their 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. As a result, other data packets cannot be forwarded.

In a broadcast domain, if Layer 2 devices forward broadcast frames repeatedly, broadcast storm will occur. The broadcast storm causes the MAC address table to become unstable, degrading the communication quality and even interrupting communication.

To prevent loops and ensure network reliability, loop prevention protocols can be configured on switches. Currently, the S series switches support the following Layer 2 loop prevention protocols:

• STP/RSTP/MSTP
• 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.

Purpose

This is a guide for technical support personnel to remove Layer 2 loops, including:

• Helping frontline service engineers describe the fault symptom and determine the scope of the fault.
• Helping TAC engineers collect NE information, analyze anomalies of NEs, and quickly locate the faulty NE and service.
• Helping R&D engineers locate the fault.

On a stably running network, the following factors may cause a fault:

• Network adjustment: such as network topology adjustment, configuration modification, and upgrade/migration
• Network environment change: such as network storm, user online behavior change (holidays, promotion activity, use of smart terminals), power/temperature change, fiber disconnection, change to daylight saving time, microwave transmission affected by weather change (rain/fog), and accident (flood/fire/earthquake/lightning)
• Network device failure: such as software bug, hardware aging (card/fiber/optical module)

The anomalies will be reflected in the traps, logs, traffic statistics, or port status on the certain NE. Therefore, to locate a fault, you need to quickly determine the fault occurrence time and fault impact scope, learn the operations that have been performed and affected NEs, and find out the faulty NE to locate the root cause.

If one or more symptoms in the following figure appear, there is a high probability that a Layer 2 loop has occurred.

Figure 6-2 Layer 2 loop symptoms