You may have used DHCP Option 43 to point an AP to its controller before. But only very few people know that Cisco APs can automatically convert themselves from the built-in controller mode (think Mobility Express or Embedded Wireless Controller) to Lightweight mode after they receive a special Option 43 from a DHCP server.
If you have a pallet of access points (or routers with built-in Wi-Fi in Mobility Express mode) next to your desk and need to convert all of them to Lightweight mode, simply configure DHCP Option 43 in the following format on your DHCP server and plug them into a PoE capable switch. After the APs boot up and receive the option from DHCP server, they automatically switch to the Lightweight mode and attempt to join the configured controller (192.168.130.2 in our case).
Option 43 format used for AP conversion
f2:05:c0:a8:82:02
“f2” tells the AP that we want it to switch to Lightweight mode
“05” means that only one controller IP address will follow
“c0:a8:82:02” is the controller IP address (192.168.130.2 in this case) in hexadecimal format, search for “IP to Hex Converter” if you do no want to do the math
Cisco IOS/IOS-XE DHCP server configuration
You can run DHCP server on a Catalyst switch. The DHCP scope configuration is straightforward.
ip dhcp pool <pool name>
network <ip network> <netmask>
default-router <default-router IP address>
dns-server <dns server IP address>
option 43 hex f205c0a88202
WLAN Pi, Raspberry Pi and any other Linux ISC DHCP server configuration
Special thanks to Nicolas Darchis, who helped me find the “vendor-encapsulated-options” option. It lets you enter Option 43 in the hex format and all it takes is a single line of DHCP server configuration.
# eth0 DHCP scope on ISC DHCP server
subnet 192.168.130.0 netmask 255.255.255.0 {
interface eth0;
range 192.168.130.100 192.168.130.200;
option routers 192.168.130.1;
option domain-name-servers 208.67.222.220, 208.67.222.220;
default-lease-time 86400;
max-lease-time 86400;
option vendor-encapsulated-options f2:05:c0:a8:82:02;
}
DHCP server on Cisco Meraki MX appliance
If your DHCP server runs on a Cisco Meraki MX appliance, you can easily configure Option 43 using Dashboard. Here are the instructions.
Packet capture or it did not happen
Here is the DHCP Offer packet with the special Option 43 value sent from DHCP server to the APs. They will start the conversion automatically after receiving it.
Verify successful AP conversion to Lightweight mode
Console to one of the APs and you will notice this message:
[*08/25/2020 23:24:39.5620] Last reload reason : 2: AP type changed from ME to CAPWAP
Or you can let the AP finish its job. And then verify successful conversion to Lightweight mode whenever you are ready using the “show version” command.
9120#show version
<output omitted>
9120 uptime is 0 days, 0 hours, 5 minutes
Last reload time : Tue Aug 25 23:24:39 UTC 2020
Last reload reason : AP type changed from ME to CAPWAP
<output omitted>
