Do all Cisco Lightweight Access Point support Option 43 in HEX?

[tuanquangnguyen]

Hi community,

I am about to deploy roughly 1000 APs of different models (old 1000 series, Aironet, Catalyst, etc., all Cisco) in geo-dispersed braches. Each branch has their own Windows DHCP servers where there would be IP reservation for all access points.

I was thinking of specifying Option 43 in HEX as I normally do. But, some colleagues came across the configuration guide where they would specify user-defined VCI then sub-option 102 (or 241) for 1000 and Aironet, respectively. The thing I find this type of configuration unnecessary is:

• All AP's management IP address are of the same subnet (same DHCP scope)
• No user/workstation get the IP addresses in this scope - due to our LAN segmentation design and DHCP reservation
• Not all AP are of the same models within the same branch

Therefore I would like to ask if I just specify Option 43 in HEX, would all AP operate normally?

Thanks in advance.

[Leo Laohoo · Accepted solution]

Cannot say about Meraki or SMB APs, but all Cisco support DHCP Option 43.

[tuanquangnguyen]

Well as I have mentioned, we only have Cisco 1000 series, Aironet and Catalyst.

Thanks for your reply.

[Leo Laohoo]

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@tuanquangnguyen wrote:

Well as I have mentioned, we only have Cisco 1000 series, Aironet and Catalyst.

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Recommended is using DHCP Option 43 but DNS can also be used.  

[tuanquangnguyen]

Hi @Leo Laohoo,

Just another question. On Windows DHCP, is the Type's hex supposed to be 0xF1 (f1) or 0x00 0xF1 (00f1)?

I came across guides that seemed to mention different hex for the Type part

YouTube - this video mentioned the Type to be 0xF1 

This guide, on the other hand, specified it to be 0x00 0xF1 (00f1) instead 

[Scott Fella]

Take a look at some online calculators for option 43.

f104 - single controller

f108 - two controllers

https://shimi.net/services/opt43/

[tuanquangnguyen]

Hi @Scott Fella,

I know well about how to calculate Option 43. My question was about the way to configure it on Windows DHCP Server, whether to append a 0x00 before the 0xF1 part or not.

As seen in my previous post, the YouTube video mentioned that the Hexa part only needs 0xF1, while the guide underneath stated that on Windows DHCP Server it needs to be 0x00 0xF1. In this Cisco technote, the photo also shows the Binary to be 0x00 0xF1 0x08 etc. - but if you calculate Option 43 it should only be 0xF1 0x08 etc.

[Scott Fella]

Well just follow the guide and take a look at other guides out there. Here is one for an example.

https://networkguy.de/cisco-wlc-dhcp-option-43/