In which situation can ARP entries be learned after strict ARP learning is enabled

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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.

Other related questions:
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.

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.

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.

ARP learning rate limiting on S series switch
The S series switches, except S1700, use different ARP learning rates: S series fixed switches: The default CIR value for ARP request and reply packets is 64. Assume that the packet length is 60 bytes. The ARP learning rate can reach 100 pps. When the CIR value is increased to 500, the rate can reach 200 pps, with the CPU usage lower than 60%. The rate of ARP learning triggered by ARP Miss packets is lower than 50 pps. The tested rate is 30 pps. S series modular switches: A test assumes that the default CPCAR settings (128 kbps for MPU and 64 kbps for LPU) are used, the CPU usage is lower than 50%, and the packet length is 60 bytes. The tested rate of ARP request and reply packets is 100 pps. When the CPCAR values are increased, the rate of received ARP request packets is 1000 pps and the rate of received ARP reply packets is 500 pps. The rate of ARP learning triggered by ARP Miss packets is lower than 50 pps. The tested rate is 30 pps.

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.

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