How to determine whether loops occur on a Layer 2 network

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To determine whether loops occur on a Layer 2 network, two methods are available:
Run the display interface brief | include up command to view traffic on all Up interfaces. If a loop occurs, the values of InUti and OutUti approximate to 100% when you run the command multiple times, which are greater than the service traffic.

Command output upon the first query is as follows:
[HUAWEI] display interface Ethernet brief | include up
InUti/OutUti: input utility/output utility
Interface PHY Auto-Neg Duplex Bandwidth InUti OutUti Trunk
GigabitEthernet0/0/1 up enable full 1000M 0.56% 0.56% -- Command output upon the last query is as follows:
[HUAWEI] display interface Ethernet brief | include up
InUti/OutUti: input utility/output utility
Interface PHY Auto-Neg Duplex Bandwidth InUti OutUti Trunk
GigabitEthernet0/0/1 up enable full 1000M 76% 76% -- Determine whether MAC address flapping occurs on the switch.
Run the display mac-address command. If different interfaces of the switch learn the same MAC address, MAC address flapping occurs.
Configure MAC address flapping detection on the switch. If MAC address flapping is detected, the switch generates an alarm. For details about how to configure MAC address flapping detection, see How Do I Configure MAC Address Flapping Detection?

Other related questions:
Query MAC address flapping records on S series switch
When loops occur on S series switches (except S1700), MAC address flapping often occurs. You can check whether MAC address flapping occurs on a switch to determine whether a loop occurs on the switch. Methods to check MAC address flapping vary depending on the system version. 1. In versions earlier than V200R001 on S series switches: Run the display trapbuffer command. If the command output displays MAC move detected, MAC address flapping occurred on the switch. 2. E series switches and S series switches running V200R001 and later versions: - Run the display trapbuffer command. If the command output displays MAC move detected, MAC address flapping occurred on the switch. - Run the display mac-address flapping record command. If information is displayed, MAC address flapping occurred on the switch.

How to check whether forwarding is looped on a Layer 2 network
The following events indicate that forwarding is looped on a Layer 2 network: Massive congestion and full-bandwidth storm. Run the display interface ethernet brief | include up command to check whether the incoming and outgoing traffic of a port reaches the maximum. Massive MAC address flapping is found during MAC flapping detection. Run the display trap logbuffer command to check whether MAC address flapping alarms are generated.

How can I check whether a loop exists on an S series switch
The simplest way to check the existence of a loop on an S series switch (except the S1700) is to check whether a MAC address flapping alarm exists on the switch. If the alarm exists, you can quickly locate the loop based on the information in the alarm. Run the display mac-address flapping record command to check whether MAC address flapping exists, or run the display trapbuffer command to check whether the alarm contains The mac-address has flap value or MAC move detected. For more information, visit Layer 2 Loop Troubleshooting.

How to determine whether an S series switch is a Layer 2 or Layer 3 switch
All models of the S600, S1720, and S2700 are Layer 2 switches. All models of the S3700 and S6700 are Layer 3 switches. The S5700-LI, S5700S-LI, S5710-LI, S5720-LI, and S5720S-LI series are Layer 2 switches. The S5700-EI, S5700-SI, and S5700-HI series are Layer 3 switches. All models of the S9300, S7700, S9700, and S12700 are Layer 3 switches.

How do I check whether LSP flapping occurs on an AR
You can run the display mpls lsp command to check the number of LSPs. If the number of LSPs changes frequently, LSP flapping occurs. In this case, run the snmp-agent trap enable feature-name mpls_lspm command. If the alarm about LSPs is frequently generated, LSP flapping occurs. Run the undo snmp-agent trap enable command. For example: #Jul 25 22:49:18 2020 HUAWEI LSPM/5/TRAP:1.3.6.1.2.1.10.166.2.0.2 LSP 210958 Changes to Down #Jul 25 22:49:19 2020 HUAWEI LSPM/5/TRAP:1.3.6.1.2.1.10.166.2.0.1 LSP 210958 Changes to Up The preceding information shows that the LSP alternates between Up and Down.

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