Monday, September 7, 2015

The growing importance of DNS in your CUCM configs

Hi Guys!

It used to be that DNS was an afterthought for most CUCM deployments, many engineers advocated at one point or another just using an IP address rather than dealing with the system team who could either be extremely cooperative or extremely slow.

Those days are GONE, my friends. 

Every bit of UC software you use should be properly configured with DNS.

 Infact, anywhere you CAN use DNS, you should be using it.

The main driver for this is definitely Jabber, I can't count the number of things that can go wrong with Jabber if you do not have your DNS configured correctly.

If you absolutely must get a system up very rapidly and the sysadmin team have been very slow to get there side finished, you can use a router as a temporary DNS server:

The config for this is shown below:

conf t
 ip domain-name
ip  dns server
ip name-server

ip host srv 0 10 8443
ip host cucm01
ip host unity

ip host imp
ip host srv 0 10 8443

As you can see from the example above, you can use the host command to specify manual DNS entries, you can even do SRV records etc, the only records i can't seem to see are CNAME records, but for our purposes this does not matter.

You can then point your UC servers to this router as there DNS server as a temporary workaround while you wait for the system administration team.


  1. Just bear in mind that changing your DNS settings will change your licensing MAC.

  2. Hi Peter,

    I just found your blog while trying to troubleshoot an issue with a new user and their EM Home-cluster problems on a single-cluster site. I jsut want to say thanks for that post! I'm doing my first build of a CUCM, CUC and IMP cluster and was wondering why you find it best to keep the DNS entries? What are the benefits here over going straight IP? My colleague who has done these for many years stated specifically to alter the DNS entries for the various enterprise services on CUCM so I'm very interested in your methodology here.

    Thanks very much again,


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  4. Awesome..You have clearly explained ...Its very useful for me to know about new things..Keep on blogging..
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  6. Hi Peter

    I wrote a longer post, but seems to have been lost...

    On a recent deployment, I was fortunate to have an extended engagement with the Cisco BU dial plan team when completing a mult-cluster design. Even in 11.x, CUCM does not support DNS caching. So for SIP Trunking in CUCM, multiple dotted IP Addresses are probably still the way to go! Only time this changes will obviously be with a secure connection where certificates come into play.

    That being said, I pretty much use DNS SRVs on all CUBE dial-peers, IM and Presence etc as far as I can. Don't think I've deployed a cluster without DNS for ages.

    CNAMEs also are great, and as best practice with CVD. Great for end users with a simple "www" address pointing to the pub.


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  8. Thanks for the excellent article Peter. Just to clarify are you saying in CUCM under System/Cisco Unified CM the best practice is to leave that as a hostname and not change it to the IP address? What about the URLs in the Enterprise Parameters? Also leave that as hostnames? I have always configured DNS on CUCM (DNS name and DNS server IP) but I remove the reliance on DNS from the Cisco Unified CM and the Enterprise Parameters. Will this cause an issue with Jabber?



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