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of information online on how to setup option 43 for Lync/Skype for Business, however, lots of this information is outdated, and many have found that their FQDN for their Lync certificate server is too long for Cisco DHCP and gets truncated, causing it not to work.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 ......you 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 servername.domain.com

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
 network 10.192.12.0 255.255.255.192
 domain-name corp.eblend.local
 dns-server 10.193.8.124 8.8.8.8
 default-router 10.192.12.1
 option ext 43 hex YOURLONGHEX43STRINGHERE
 option 120 hex YOUROPTION120STRINGHERE
 lease 8

Hope this was helpful to someone.



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  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!

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