When the AR2220 used as the AC uses the Layer 3 interface as the downlink interface to connect to an AP, the AP fails to log in

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Root cause analysis: Interconnectivity of the Layer 3 network from the AP to the AR is normal. The AR discards Discovery request packets sent from the AP.
Summary and suggestion: When the AR serves as the AC, you are advised to use the Layer 2 interface. There is no Layer 2 card on the tested AR, so the preceding networking can exist. To locate the problem, you need to be clear about the packet interaction process, and then obtain packets to find out the phase where the problem occurs. The AP and AC have routes to each other, so the AC can be pinged from the AP at Layer 3. The Discovery request packet is transmitted by the S5700 to the AR through the Layer 2 network. When the outer VLAN tag is removed, the packet still carries a VLAN tag. Therefore, the packet is dropped by the AR.

Other related questions:
What are the differences of using a Layer 3 interface and a VLANIF interface
On some models of S series switches, you can run the undo portswitch command in the interface view to switch Layer 2 mode to Layer 3 mode. Both Layer 3 interfaces and VLANIF interfaces can provide Layer 3 services. They have the following differences: 1. When an interface works in Layer 2 mode, run Layer 3 services on VLANIF interfaces. Layer 2 services are run on Layer 2 interfaces. 2. After an interface is switched to Layer 3 mode, this interface is similar to a Layer 3 interface of a router, and can only provide Layer 3 services. To run Layer 3 services, use the Layer 3 or VLANIF interface. However, to run both Layer 3 and Layer 2 services on the same interface, use the Layer 2 mode to provide Layer 2 service and VLANIF to provide Layer 3 service.

What differences exist in the way APs discover an AC when going online on Layer 2 and Layer 3
When APs go online on Layer 2 or Layer 3, the differences in the way the APs discover an AC are as follows: - When going online on Layer 2, APs discover an AC by using broadcast. - On Layer 3, APs can go online in Option 43 mode or DNS mode. - In Option 43 mode, APs obtain their IP addresses and also the IP address of an AC, and then discover the AC by using unicast. - In DNS mode, APs obtain their IP addresses and also the IP address of the DNS server and the AC domain name. Then, the APs send requests to the DNS server to parse the AC domain name. As a result, the APs obtain the IP address of the AC and discover the AC by using unicast.

What are the differences in APs' Layer 2 and Layer 3 access to ACs
- In Layer 2 access mode, an AP broadcasts a probe frame to discover ACs. - In Layer 3 access mode, an AP discovers ACs using DHCP Option 43 or DNS. a) When Option 43 is used, an AP obtains an AC's IP address when the AP obtains its own IP address. Then

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