Actual combat case: AC cannot detect AP. Can’t it solve such a basic problem?

The case shared in this issue is about wireless network related issues.

1. Problem Background
A coal group uses Cisco three-layer switches as core and aggregation. The wireless uses TP AC+AP networking connected to the core switch. As a result, the AC cannot find the AP. What should I do?

Topology:

2. Troubleshooting ideas
(1) Determine whether the network parameters of the AC are normal and whether the AP can obtain the corresponding IP address normally;

(2) Determine whether the AC can ping the AP;

(3) Confirm whether AP online authentication is enabled in AC. Huawei/H3C has MAC address authentication as follows;

(4) Confirm whether AC and AP are in the same broadcast domain. If so, AC can automatically identify AP through broadcast (same as NVR discovering IPC). If not, if the two are in different VLANs, you need to set option field unicast discovery;

3. Basic analysis

Step 1: Confirm whether the network parameters of AC and AP are normal

AC is checked and no problem is found. Check from the core DHCP client list.


The IP of AP is also correct.

But please note that AC and AP are not in the same VLAN segment.

Step 2: Confirm that AC can ping AP and whether authentication is enabled

Ping AP in AC diagnostic tool is successful:

Step 3: Check whether option field is configured in DHCP server

Because AC and AP are in different segments, they cannot be automatically discovered through broadcast, so DHCP server (Cisco core switch) needs to send option field to AP to realize unicast discovery, so let's take a look at the relevant configuration:



Therefore, the TP AP cannot unicast to find the AC at 192.168.100.253 to be managed and used.

Problem summary and solution
Problem summary:

The root cause of the problem is that there are three layers of switches (which can be understood as routers) between the AP and the AC, which blocks the process of the AC automatically broadcasting to find the AP. However, the core switch is not configured with an option field, which makes it impossible for the AP to find the AC to be managed in a unidirectional manner.


Solution: 

By configuring the DHCP server of the Cisco switch, add Option60 and Option138 fields (TP-LINK uses these two, Huawei/H3C uses option43 and 138). The relevant configuration is as follows: 

FAQ: 

Q: What is the function of Option60?



A: This field is mainly a manufacturer's logo. AP uses this field to identify available DHCP servers. The DHCP server of the layer 3 switch can be changed to a DHCP server recognized by AP.

Q: What is the function of Option 138?

A: This field is used to inform AP of the location of AC in the network and let AP know the IP address of AC. After AP startup is completed, it communicates directly with the IP address. Setting DHCP OPTION 60 and 138 will not affect the use of the terminal.