Tag Archives: WLC

Option 43 Cisco WLC discovery

When your AP boots, it will first discover which controllers are available.
One of the techniques is through its received DHCP options.

Two specific options are used here;

Option 60 and Option 43

Option 60
If you want this subnet only to be used by a certain AP type, you can use this option to check for the “Vendor Class Identifier”(VCI) string.
A list of Cisco-specific VCI strings can be found here.
If you want any device to be able to get a DHCP offer in this subnet, you can just leave out option 60.

Option 43
If you want the AP’s to discover Wireless Lan Controllers through DHCP, you’ll have to put the IP’s of your controllers in here.
This option has a Type-Length-Value (TLV) encoding.

Type
The type is always 0xf1

Length
This is based on the number of IP addresses you want to put into DHCP Offer times 4.
So, if you put 1 address in there, this value would be 0x04.
If you have 2 IP addresses, it will be 0x08 (2 times 4 = 8 and 8 in HEX is 0x08).
If you have 6 IP addresses, it will be 0x18 (6 times 4 = 24 and 24 in HEX is 0x18).

Value
This is based on your IP address.
You should convert every decimal octet to its Hexadecimal values.
If you have a controller at 172.16.20.1, this will become ac101401 (172 = 0xac , 16 = 0x10 , 20 = 0x14 , 1 = 0x01)

So if we take all of this together, and create an example with a single Wireless Controller at 172.16.20.1, this will give us: f104ac101401

If you do a “dhcp debug detail” on your AP, you will see the value:
*MarĀ  1 00:05:40.027: DHCP: Scan: Vendor specific option 43: F104AC101401

For multiple controllers, the secondary value simply needs to be appended,
e.g. if you have a secondary controller at 172.16.20.2, the value needs to be:
f108ac101401ac101402.

Hexadecimal,Decimal,Binary conversion can be done here.