IP-Phones with DNS configuration

[Pascal Blumenthal]

Hello

We are planning to implement Cisco Jabber and our actual infrastructure with CUCM Version 10.5.2 works with pure IP, I mean, all servers are configured with IP Adress and not with the Host Names.

Now, we have to change this and of course, the DHCP configuration need to be adapted so the IP Phones gets the IP's from our DNS servers to resolve the Host Names of our CUCM servers to get the config file and to register to the CUCM servers.

My question is, how oftenand in which status/case the IP Phones will contact the DNS servers? For example when a phone requests the config file, place a call and or receive a call etc.

Does the phone have a DNS "cache" or something similar like a Windows Workstation?

I couldn't find any information about that.

Any help will be appreciated.

Thank you very much.

Kind regards,
Pascal

[Vivek Batra]

Now, we have to change this and of course, the DHCP configuration need to be adapted so the IP Phones gets the IP's from our DNS servers to resolve the Host Names of our CUCM servers to get the config file and to register to the CUCM servers.

If you've been changed hostname to IP address in CUCM configuration wherever required, you don't need to worry anymore. So the phones will only get IP address in TFTP configuration and never go to DNS for domain resolution.

- Vivek

[Pascal Blumenthal]

Hi Vivek

Thank you for your answer.

We want to have the opposite, I mean we want to change from IP to Host Names.

In this case, I guess that the phones get the TFTP Server Name with the Option 150 in the DHCP Request. At this time, the Phone needs to resolve this and after that, the Phone requests the config file from the TFTP Server with the CUCM Host Names to register on it. To resolve the Names with the IP Address, the Phone will contact the DNS Server.

Once the Phone is registered, does the Phone sends any more Requests to the DNS Server in case of placing a call or receiving a call or if you Login on the Phone with Extension Mobility etc.

I like to know how often and in which cases the Phone starts a DNS Request.

Cheers,
Pascal

[Vivek Batra]

I believe that any standard operating system should honor Time to live value recieved in DNS response and same should be adopted by phone too.

I've found one older bug associated with 7960 phone which implictly meant that phone entertains TTL value. Descirption is as follows;

Problem: Cached DNS entry causes failures in a farmed proxy server environment (CSCds76511)

Description: The Cisco SIP IP phone only caches the first IP address that is returned from a DNS lookup. The phone does not attempt to use any other IP address when the first IP address is unreachable. This might causes problems when a server farm with multiple IP addresses is used and the proxy server with the first IP address is down.

Recommended Action: Use a shorter Time to Live (TTL) value for the DNS entry.

[Ayodeji Okanlawon]

Pascal,

You are indeed correct that deploying applications such as Jabber requires a cluster that is DNS centric. Here are my thoughts on your questions

1. option 150 can and it should still be setup using ip address. There is no need for you to change this. This has no impact on jabber etc. So keep this as it is.

2. Phones will need DNS for the following possible scenarios (there could be more)

  2.1 Resolving CUCM FQDN once it has downloaded its config from TFTP

  2.2 Services lookup (EM, corporate directory etc)

3. ITL consideration. When you change the host name on your CUCM servers, your certs are automatically autogenerated if you are using self signed certs. The implication of this is that when you have phones that have already downloaded ITL files, then they will no longer trust any configuration file downloaded from these CUCM. You need to plan this carefully as it could burn you really badly ( I have recently gone through this pain). Please refer to the link below and pay close attention to the section on "changing host names and domain names"

https://supportforums.cisco.com/document/68701/communications-manager-security-default-and-itl-operation-and-troubleshooting

Summary of this, do not change the hostname on your server at once. Do it once by one and once one server is done, ensure you reboot all your phones so they can download new ITL files and still have a TVS server they trust to verify the new ITL files

4. Ensure you add the domain name to your CUCM servers. This is vital if you want to deploy Collaboration edge in the future

5. When you configure your service profiles for Jabber ensure you use FQDN of your IM and P server. Unity connection server etc. This is because Jabber will throw cert errors if you use IP here instead of the name contained in the CN of your certs

[Pascal Blumenthal]

Hi Ayodeji oladipo,

thank you very much for your answer. I guess we will have a good overview right now.

I wish you a nice day.

Kind regards,
Pascal

[Ayodeji Okanlawon]

Pascal,

Glad to help. Don't forget to rate the post or mark it as answered . It helps others in the future to know the correct response to similar questions

[rramlal]

Hi Ayodeji,

I am seeking your advice, since based on your response, it seems you have experience implementing CUCM and CUC installs using DNS.

For CUCM, i need to build a cluster across a WAN with jabber and E-SRST manager. Based on my research, DNS is recommended for this requirement. I would like to know if it is also required for CUC cluster across a WAN as well. I plan to use a SIP integration between CUC and CUCM. 

Would it also makes sense to use the fqdn for the sip trunk between CUC and CUCM and also CUCM and a voice GW?

I would really appreciate your feedback, thanks.

[Ayodeji Okanlawon]

Hi,

Yes you should FQDN for CUC as well. Dont forget that Jabber needs voicemail access too and using FQDN will reduce users getting cert errors..

No, there is no need to use FQDN on SIP trunks

Regards

[rramlal]

Hi,

Thanks for your swift response.

When you say, SIP trunks dont need FQDN, what about the SIP trunk between CUC and CUCM? Can I use the IP address of the CUC on the SIP trunk?

[Lkb240001]

When I hook it up both phones up to the network work there's no dial tone please let me know thank you