Why does an ARP entry not contain VLAN information

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If an ARP entry does not contain VLAN information, the entry contains information about a Layer 3 interface. If an ARP entry contains VLAN information and the interface in the entry is not a sub-interface, the entry contains information about a Layer 2 interface.
Run the display arp command to view ARP entries. For example, the ARP entries in V100R003C00 are as follows:
- When an ARP entry contains VLAN information, a VLAN ID is displayed in the VLAN field in the command output.
- The ARP entry of which the IP address is 10.1.1.2 and the MAC address is 04f9-388d-e685 is dynamically learned from 10GE1/0/3, the VLAN ID is 101, and the remaining lifetime of the entry is 5 minutes.
- The ARP entry of which the IP address is 10.1.1.3 and the MAC address is 0023-0045-0067 is a static entry, the outbound interface is 10GE1/0/3, and the VLAN ID is 101.
display arp network 10.1.1.0 24
ARP Entry Types: D - Dynamic, S - Static, I - Interface
EXP: Expire-time

IP ADDRESS MAC ADDRESS EXP(M) TYPE/VLAN INTERFACE VPN-INSTANCE
------------------------------------------------------------------------------
10.1.1.3 0023-0045-0067 S/101 10GE1/0/3
10.1.1.1 306b-2079-2205 I Vlanif101
10.1.1.2 04f9-388d-e685 5 D/101 10GE1/0/3
------------------------------------------------------------------------------
Total:3 Dynamic:1 Static:1 Interface:1

- When only the ARP entry type is displayed in the TYPE/VLAN field, the ARP entry does not contain VLAN information.
ARP Entry Types: D - Dynamic, S - Static, I - Interface
EXP: Expire-time

IP ADDRESS MAC ADDRESS EXP(M) TYPE/VLAN INTERFACE VPN-INSTANCE
------------------------------------------------------------------------------
172.16.1.3 0023-0045-0069 S 10GE1/0/13
172.16.1.1 306b-2079-2202 I 10GE1/0/13
172.16.1.2 0025-9e95-7c32 3 D 10GE1/0/13
------------------------------------------------------------------------------
Total:3 Dynamic:1 Static:1 Interface:1

Other related questions:
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.

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.

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