Loopback detection usage scenario of S series switch

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S series switches (except S1700) can detect self-loops on an interface, loops on the downstream network, and loops between interfaces.
1. Detect self-loops on an interface.
During network deployment, Tx-Rx (fiber transmit end and fiber receive end) self-loops often occur on interfaces. For example, Tx-Rx self-loops often occur when the fiber is inserted incorrectly or the local interface is damaged by high voltage. A self-loop occurs on the interface of the Switch; as a result, packets sent by the interface are sent back to this interface. This may cause problems such as abnormal traffic forwarding and MAC address flapping.
2. A loop occurs on the downstream network or device of the Switch. As a result, packets sent by Interface1 are sent back to this interface after passing through the downstream network or device of the Switch.
3. A loop occurs on the network or between interfaces of the Switch. As a result, packets sent by Interface1 are sent back to Interface2.

Other related questions:
Configure single-interface loopback detection on S series switch
How to configure single-interface loopback detection on S series switches (except S1700)? Among external loopback detection, only single-interface loopback detection is supported on switches. After external loopback detection is enabled on an Ethernet interface of a switch, the switch periodically checks whether external loopback occurs on each Ethernet interface. If an S series switch finds that external loopback occurs on an Ethernet interface, by default, it blocks the interface when it runs a version earlier than V200R001 or shuts down the interface when it runs V200R001 or a later version. If an E series switch finds that external loopback occurs on an Ethernet interface, by default, it shuts down the interface. 1. Application Scenario In actual networking, single-interface loopback detection is often used on a downlink interface of a newly deployed or added access switch. You are advised to set the action taken after loopback is detected to block. 2. Configuration Procedure Enable loopback detection globally. This function will then be enabled on all interfaces simultaneously. [Huawei] loopback-detect enable Modular switches running V200R001 or later support loopback detection in a maximum of eight VLANs. Fixed switches running V100R005 or later support loopback detection in a maximum of eight VLANs. E series switches support loopback detection in a maximum of eight VLANs. The following example describes how to configure loopback detection on a fixed switch: [Huawei-Ethernet0/0/1] loopback-detect packet vlan 20 21 22 23 24 25 26 27 [Huawei-Ethernet0/0/1] loopback-detect action block Among E series switches, modular switches running V200R001 and fixed switches running V100R005 or later save loopback alarm information in the trap buffer, and the alarm information can also contain information about the VLAN in which loopback is detected. Alarm information is as follows: #Jan 1 2008 06:43:54-08:00 Quidway LDT/4/Porttrap:OID1.3.6.1.4.1.2011.5.25.174.3.3 Loopback does exist on interface(5) Ethernet0/0/1 ( VLAN 20 ) , loopback detect status: 4.(1:normal; 2:block;3:shutdown; 4:trap; 5:nolearn) 3. Precautions Loopback detection will consume some system resources. After deploying or adding access switches, run the undo loopback-detect enable command to disable this function.

Will logs be generated when loops are detected on S series switch' interfaces where loopback detection is enabled
Logs and alarms will be generated after loops are detected on modular switches' interfaces where loopback detection is enabled. To view the related alarms, run the display trapbuffer command. Only alarms will be generated after loops are detected on interfaces of fixed switches (except S1700). To view the related alarms, run the display trapbuffer command.

Can loopback detection be used together with VLAN-Mapping on interfaces of S series switch
When VLAN-Mapping is configured on interfaces of S series switches (except S1700), only loops in the translated VLAN can be detected, and loops in the original VLAN cannot be detected. The reason is that: When a switch receives a packet, it will replace the VLAN ID carried in the packet with the translated VLAN. If the VLAN configured in loopback detection is the original VLAN, the switch finds that the VLAN where a loop is detected is not the original VLAN and will consider that no loop exists.

View CPU usage and memory usage on s series switch
For S series switches except S1700 switches, you can run commands to display CPU and memory usage. - display cpu-usage [ slave | slot slot-id ]: displays CPU usage statistics. - display cpu-usage configuration [ slave | slot slot-id ]: displays CPU usage configuration. - display memory-usage [ slave | slot slot-id ]: displays memory usage statistics. - display memory-usage threshold [ slot slot-id ]: displays the alarm threshold for memory usage. For S series switches and E series switches except S1700 switches, you can check CPU and memory usage through the related MIB OIDs. - hwEntityCpuUsage 1.3.6.1.4.1.2011.5.25.31.1.1.1.1.5: provides CPU usage. - hwEntityCpuUsageThreshold 1.3.6.1.4.1.2011.5.25.31.1.1.1.1.6: provides the alarm threshold for CPU usage. - hwEntityMemUsage 1.3.6.1.4.1.2011.5.25.31.1.1.1.1.7: provides memory usage. - hwEntityMemUsageThreshold 1.3.6.1.4.1.2011.5.25.31.1.1.1.1.8: provides the alarm threshold for memory usage.

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