Strict ARP learning is enabled on S series switches, and the user has learned the switch's ARP entry. Why cannot the switch learn the user ARP entry by pinging the user

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For S series switches:
After strict ARP learning is enabled, the switch learns ARP entries only from the Reply packet sent in response to locally originated ARP Request packets.
The firewall installed on the PC may prevent the PC from sending ARP Reply packets when receiving ARP Request packets, or the NIC on the computer cannot return ARP Reply packets. In this case, the switch cannot receive ARP Reply packets no matter whether the switch sends ping packets to the user or the user sends data packets to the switch to trigger ARP Miss messages. Therefore, the switch cannot learn the user's ARP entry.
If this problem occurs on only a few users, configure static ARP entries for the users; if the problem happens on most users, disable strict ARP learning on the switch.

Other related questions:
Reason why S series switch cannot learn ARP entries
When an S series switch, except S1700, works at Layer 2, the switch does not have ARP entries and cannot learn ARP entries. When an S series switch, except S1700, works at Layer 3 and cannot learn ARP entries, rectify the fault as follows: (1) Possible cause: The link between the switch and connected device fails. Solution: Perform ping operations to check whether the link fails. If so, rectify the link failure. (2) Possible cause: ARP strict learning is enabled on the switch. (After this function is enabled, the switch learns only the ARP reply packets in response to the ARP request packets sent by itself.) Solution: Run the undo arp learning strict command in the system or interface view to disable ARP strict learning. (3) Possible cause: The switch has too many ARP entries and may suffer an ARP attack. Solution: Configure static ARP entries for key servers or users and enable attack defense policies. Note: (1) By default, ARP strict learning is enabled on some models among fixed switches and disabled on modular switches. When a fixed switch connected to a modular switch receives a gratuitous ARP packet, the fixed switch does not learn ARP entries. Therefore, some fixed switches cannot learn ARP entries. (2) After ARP strict learning is enabled on a switch, the switch actively sends ARP request packets to hosts. Some PCs with wireless network adapters installed do not respond to ARP requests, so the switch cannot learn the ARP entries of the connected PCs. The PCs respond only after the network adapters are restarted. In this situation, disable ARP strict learning.

In which situation can ARP entries be learned after strict ARP learning is enabled
If strict ARP learning is enabled, a device does not learn ARP entries matching source IP addresses after receiving ARP request packets. The device updates an ARP entry matching the source IP address in a packet only when the device receives an ARP reply packet, the destination IP address of the ARP reply packet is the device address, and there is an ARP entry, temporary ARP entry, or actual ARP entry matching the source IP address. The device must send an ARP request packet to the source end. After receiving an ARP reply packet, the device learns the ARP entry matching the source end.

How to configure strict ARP entry learning on S series switches
For S series switches (except S1700 switches), enabling strict ARP entry learning allows the switch to learn only ARP entries for ARP Reply packets in response to ARP Request packets sent by itself. Enabling strict ARP entry learning on the switch device affects only the ARP entry learning on the switch, not on the hosts. # Enable strict ARP entry learning on VLANIF 100. [HUAWEI] interface vlanif 100 [HUAWEI-Vlanif100] arp learning strict force-enable # Enable strict ARP entry learning globally on the switch and enable this function on GE1/0/1. [HUAWEI] arp learning strict [HUAWEI] interface gigabitethernet 0/0/1 [HUAWEI-GigabitEthernet0/0/1] undo portswitch [HUAWEI-GigabitEthernet0/0/1] arp learning strict force-disable The physical interfaces on some switch models cannot switch between Layer 2 and Layer 3 modes through the undo portswitch command.

Why cannot ARP entries be learned correctly when a lot of ARP entries exist
This problem may occur on fixed switches of V200R001 and earlier versions. The reason may be that ARP attacks occur on the device. You are advised to configure static ARP or ARP anti-attack to solve this problem.

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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