Thursday 26 May 2016

Cisco IOS DHCP with LONG option 43 for Lync/Skype for Business

Hello Everyone,

So I have been bashing my head against the wall on this one for a while, so I figured I write a blog post about it so other's will hopefully not have to.

There is plenty of information online on how to setup option 43 for Lync/Skype for Business, however, lots of this information is outdated, and many have found that their FQDN for their Lync certificate server is too long for Cisco DHCP and gets truncated, causing it not to work. We have over a hundred remote sites where we deploy DMVPN routers, so using local DHCP on the Cisco router makes the configuration a million times better.

I battled with it for a long time, until I accidentally stumbled upon an extra variable available in newer Cisco IOS. I don't know exactly when it came into play, but there seems to be no mention of this anywhere on the internet.

The extra option is simply "ext", so instead of typing option 43 hex type option ext 43 hex...and your long string option 43 is accepted, and best of all, WORKS!

To summarize everything in one page, you run DHCPutil.exe off your Lync server as such:

dhcputil.exe -sipserver

You will need option 120 value in full, and option 43 value in full.

On the Cisco DCHP server, you would simply run the following to make a DHCP scope, with your own values and additional options:

ip dhcp pool DATA-POOL
 domain-name corp.eblend.local
 option ext 43 hex YOURLONGHEX43STRINGHERE
 lease 8

Hope this was helpful to someone.


  1. skype users is extremely good for your business. You can get and make free calls to your clients' or customers' or business partners' Skype and expand your business to new heights.

  2. Quick message to say thanks for the article. Found this issue happening at one of our customer sites recently, and it was a bit of a nightmare to try and fix. Deployed this solution, will see what happens!

  3. I admit, I have not been on this web page in a long time... however it was another joy to see It is such an important topic and ignored by so many, even professionals. I thank you to help making people more aware of possible issues Skype Latest Setup For Windows

  4. Thanks for sharing such informative info hope you keep sharing i personally recommend visiting entertainment company

  5. It's hard to understand the working of command prompt for beginners, but not for Buy Assignment Online UK. This firm has the experts that can help you in learn programming and can write your programming assignments.

  6. I am very glad, I have come over to this post because it is very informative and useful. However, this is my first time visiting your blogs, but It is my best experience, and I want to visit more of your blogs, but first I have to learn methods for Research Article Publication to publish my article. After getting free from it, I promise, I will read your other blogs.

  7. Keep up the wonderful piece of work, I read few posts on this internet site and I think that your blog is really interesting and holds bands of fantastic information.
    Data Mining Assignment Help - Artificial Intelligence Assignment Help - Solidworks Assignment Help

  8. With Thomas's expertise, students can present their dissertation proposal writer findings in a visually engaging manner, incorporating graphs, charts, and tables where appropriate.

  9. The basics of command prompts may seem too complicated for beginners as not to Take My Math Class -, who is experienced . This firm has specialists that they portray it for you to learn Math and Take your Math Class.

  10. Ramma Foundation Repair is your trusted partner for top-notch Foundation Repair Edmonton. With years of experience and expertise, we ensure a sturdy and stable foundation for your property's long-term structural integrity.

  11. Monster Rabbit offers premium monster rabbit honey benefits, renowned for its natural vitality-boosting properties. Sourced from the finest ingredients, our honey ensures exceptional quality and benefits for your well-being.