Why cannot ARP entries be learned correctly when a lot of ARP entries exist

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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.

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.

Why cannot a device learn the ARP entries of directly connected PCs
By default, strict ARP learning is enabled on fixed switches. A fixed switch sends an ARP request to a client and learns ARP entries only after receiving an ARP reply from the client. PCs that have wireless network interface cards (NICs) installed do not reply to the ARP requests received from the switch. As a result, the switch cannot learn ARP entries of these PCs. These PCs reply to the ARP requests only after their wireless NICs are restarted. To solve the problem, run the undo arp learning strict command in the system view to disable strict ARP learning.

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
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.

Why VLANIF interface on S series switch has too many ARP entries
If ARP strict learning is not enabled on an S series switch, except S1700, and a VLANIF interface on the switch is in a large broadcast domain, the switch learns ARP packets from any devices and generates ARP entries. The switch may need to maintain a large number of ARP entries. To address this problem, run the arp learning strict command to enable 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.

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