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.