For S series switches, how to handle the failure to learn ARP entries caused by ARP Miss packets

For S series switches, the reasons for the failure to learn ARP entries caused by ARP Miss messages are as follows:
- The rate limit on ARP Miss messages is small. This causes the switch to discard normal ARP Miss messages and fail to send ARP Request packets to the destination network depending on ARP Miss messages.
- The CPCAR value of the ARP Miss packet is small. This causes the switch to discard normal ARP Miss messages and fail to send ARP Request packets to the destination network.
- The attacker sends a large number of network scanning packets to the switch. This causes the switch to trigger a large number of ARP Miss messages, consuming CPU resources and affecting the normal processing of ARP Miss messages.

Perform the following steps to locate the fault. Save the results of each troubleshooting step so that you can provide collected information for Huawei technical support engineers if the fault fails to be rectified.
1. Run the display arp all command in the user view to check statistics about ARP entries.
If the MAC address field is in Incomplete state, the device fails to learn the ARP entry. IP address and interface information can be obtained through the ARP entry.
2. Capture the packet header on the interface used to connect to a user and check the source IP address of the ARP packet.
3. Run the display cpu-defend statistics packet-type arp-miss all command in the user view to check whether the number of the dropped ARP Miss packets is increasing.
- If the number of dropped ARP Miss packets is 0, no ARP Miss packets are discarded by the switch. ARP entry learning fails because the rate limit on ARP Miss messages is too small.
Go to step 5. Increase the ARP Miss message rate limit according to the actual network environment.
- If the number of dropped ARP Miss packets is not 0, the rate of ARP Request packets exceeds the CPCAR rate limit and excessive ARP request packets are discarded. Check whether the CPCAR value of ARP Miss messages is configured correctly.
-- If not, go to step 4. Increase the CPCAR value of ARP Miss messages.
-- If so, ARP entry learning fails because the attacker sends a large number of network scanning packets to the switch. This causes the switch to trigger a large number of ARP Miss messages, consuming CPU resources and affecting the normal processing of ARP Miss messages.
Find the attacker based on the source IP address, and check whether the user is infected with viruses. Alternatively, add the source address to the blacklist or configure a blackhole MAC address entry to discard ARP Request packets sent by the attacker.
4. Run the car command in the attack defense policy view to increase the CIR value for ARP Miss messages.
Note: Improper CPCAR settings may affect services on your network. It is recommended that you contact Huawei engineers before adjusting the CPCAR settings.
After the configuration is complete, the attack defense policy takes effect only after it is applied.
After the preceding steps are performed, if the fault persists or has been rectified but the CPU usage is high, go to step 5. Decrease the rate limit on ARP Miss messages.
5. Run the display arp anti-attack configuration [ arpmiss-speed-limit | arpmiss-rate-limit ] command to view the ARP rate limit configuration.
6. If the fault persists, collect the following information and contact Huawei technical support.
Results of the preceding troubleshooting procedure
Configuration file, logs, and alarms of the switch

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