How to check whether forwarding is looped on a Layer 2 network

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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.

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 determine whether loops occur on a Layer 2 network
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?

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.

Whether the firewall supports Layer 2 or Layer 3 forwarding
The USG2000&5000&6000 support implementing the Layer 2 forwarding function in transparent mode. When the firewall implements route-based forwarding, the Layer 3 forwarding function is used.

Are packets in a bridge group forwarded at Layer 2 or Layer 3
Packets in a bridge group are forwarded at Layer 2. Only interfaces supporting Layer 3 functions can be added to a bridge. Data in a bridge, however, is forwarded at Layer 2.

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