DHCP & CUCM

[mightyking]

Hello Everyone,

I know we cannot have more than one DHCP server configured in a CUCM cluster but how many IP address a CUCM DHCP server can handle? 

40000 IP Phones can be registered in a cluster and 7500 in a single CUCM server.

Is that a fair statement to say a CUCM DHCP server can handle a minimum of 7500 IP Addresses? The SRND recommends to use dedicated DHCP server for networks larger than1000 phones. Could that dedicated DHCP server be a CUCM server? If so what would happen in case of having a network with 30000 IP Phones? Can we have multiple CUCM DHCP servers?

In case of using a Windows DHCP server, do we need to have two dedicated DHCP servers for Voice & Data? Can a single DHCP server serve both Data & Voice networks?

Thanks,

MK

[gearbox]

MK,

Its always best practice to NOT have CUCM acting as the DHCP service if you have the option to do so.  I am not a systems guy, but windows DHCP or DHCP on a core router can handle multiple DHCP subnets with the use of IP helper addresses on the SVI. Hope this helps.

[George Sotiropoulos]

Hello mightyking,

It is highly recommended NOT to use CUCM as a DHCP server.

Concering your question "in case of using a Windows DHCP server, do we need to have two dedicated DHCP servers for Voice & Data? Can a single DHCP server serve both Data & Voice networks?" the answer is YES, a  single DHCP Server can do the job.

George

[Chris Deren]

I agree with others, if you need to worry about number of IP addresses CUCM DHCP can issue, then it's clear the environment is decent size and you should not be using CUCM as DHCP.  Use your enterprise DHCP instead.

[mightyking]

Thank you everyone for replaying.

The following was extracted from SRND. Please read the text in bold

You should configure IP Communications endpoints to use DHCP to simplify deployment of these devices. Any RFC 2131 compliant DHCP server can be used to provide configuration information to IP Communications network devices. When deploying IP telephony devices in an existing data-only network, all you have to do is add DHCP voice scopes to an existing DHCP server for these new voice devices. Because IP telephony devices are configured to use and rely on a DHCP server for IP configuration information, you must deploy DHCP servers in a redundant fashion. At least two DHCP servers should be deployed within the telephony network such that, if one of the servers fails, the other can continue to answer DHCP client requests. You should also ensure that DHCP server(s) are configured with enough IP subnet addresses to handle all DHCP-reliant clients within the network.

It sounds to me that the DHCP servers need to reside in the telephony network or have at lease two network cards one in Data network and the second one in Voice network. Please see the following link which confirms the above statment.

Using One DHCP Server for Voice and Data Networks

https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/19580-dhcp-multintwk.html

Thanks,

MK

[Chris Deren · Accepted solution]

Not exactly.  Any existing DHCP server will do just fine, you just need to make sure your voice L3 interface can get to it via ip helper no differently from data subnets, and ensure you have proper option 150 pointing to CUCM TFTP server(s) defined as either part of the scope or globally. I have been working on Cisco UC for close to 20 years and have not once seen CUCM used as DHCP, and I have seen many systems (not to say there are not any as I am sure there are).  Most commonly either MSFT DHCP, infoblox, or even Cisco router are used, among many others. What do you use today for data DHCP? 

[mightyking]

Thank your Chris,

2 more questions:

1) Do you think it would be a good design to have a dedicated DHCP server for telephony network in order to avoid layer 3 (IP-Helper) dependancies?

2) Would you recommand using a centerlized DHCP server for a clustering over WAN topology?

Thanks,

MK

[Chris Deren]

1. No, I don't see the reason.  What dependency are you alluding to? Are all of your phones on a single subnet? If you have multiple locations, etc. you would have multiple subnets and thus same dependencies. 

2. CUCM deployment has no bearing on DHCP architecture as your servers don't get IP addresses from the DHCP, only PHONEs do.  Servers need to get static IP addresses, and thus the DHCP architecture needs to be considered from the endpoint (phone) deployment perspective not the servers.  If you have local DHCP servers at each site then you those for local phones, if everything is centralized today then keep it that way with adequate lease times.

[mightyking]

We have multiple locations and multiple subnets.

The design we have today is:

2 DHCP servers in 2 separate data centers at the HQ

10 locations in 10 different cities.

Servers are using static IP addresses

Phones are using DHCP service from HQ over WAN

No local DHCP servers

Do you see any issue with this design?

Thanks,

MK

[mightyking]

Anybody?

[TONY SMITH]

Personally I think I'd want to have DHCP onsite.  Unless your business is such that it doesn't matter if the phones go off if the WAN is down, for example not using SRST or local gateway or subscriber.

I wouldn't necessarily separate DHCP for voice and data unless the data network is separately administered.  

[SaibhuwanSudina70631]

 Hi Chris,

I am trying to register a 8865 video ip phone on CUCM 12.0, but it is showing the status as "Rejected".

The issue is like i have four phones of 8865 model and one was successfully registered, one is showing "Rejected" and two are not getting registered. ( error is "Telephone service unavailable").

could you please help me out with this situation...

Thanks & Regards

Sai Bhuwan

[mightyking]

Hi,

Does the phone receive an IP Address?

Make sure the following steps are completed:

- First line is configured with an extension

- Using the correct phone modle in CUCM Config page

-  Perform a factory reset on the phone.

On the switch where the phone is connected, Do "Show CDP neighbor to confirm

- The phone model,

- IP address & VLAN. and compare the config with the CUCM config

MK

[SaibhuwanSudina70631]

Dear mightyking,
Thanks for your response. i am able to "Register" the phone successfully, after performing a cluster restart.

Thanks & Regards
Sai Bhuwan.

[SaibhuwanSudina70631]

Hello

Please suggest me what yo do in the below scenario.

we have few grand stream phones and few Cisco IP phones.

Now, there is also a CUCM, which i am managing and....a third party unknown IP-PBX ( i don't know the model or name ), which is located remotely.

now grand stream phones are registered on the IP-PBX, where it is located remotely. and Cisco IP-Phones are registered by me on the CUCM which is present it the same office.

now the scenario is like....

I have created a SIP trunk on CUCM with the destination IP of that remote unknown IP-PBX. SO CUCM was successfully communicating with remote unknown IP-PBX.

I don't know what they have configured, but grand stream phones are able to communicate with Cisco Ip Phones with an extension of 8888 ( eg : 88882001 ). but when i tried to make a call from Cisco IP phones to Grand stream phones...., the call is not getting placed.

Now, the requirement of customer is like, call processing should be happening between, 

1) Cisco phones - Cisco phone ( working fine )

2) Grand stream phones - Grand stream Phones ( working fine )

3) Grand stream phone - cisco phones ( working fine ) ( with extension of 8888 )

4) Cisco phones - Grand stream phone ( solution required ) ( please suggest what to do )

Thanks & Regards

Sai Bhuwan