IP phones and DHCP/DNS

October 25, 2010 at 11:24:44
Specs: Windows XP
I am attempting to implement an IP phone system at my company and am having some difficulty understanding the most sensible way to implement DHCP and DNS for this system.

I have two subnets for example 1.1.1.x and 1.2.1.x, 1.1.1.x is for data, and 1.2.1.x is for voice.

I have one DHCP server on the 1.1.1.x subnet, and a sip proxy server with a NIC attached to 1.1.1.x and a separate NIC attached to 1.2.1.x.

All of my equipment (PCs, phones, servers, printers etc.) is attached to a single PoE switch. At the office side a wire coming from a wall jack connects into the IP phone, and another wire coming out of the IP phone connects to the PC to give both devices access to the network. (This is done to avoid having to rewire the building with an extra port in each office for IP phones and buy an additional switch explicitly for IP phones.)

The first part of this question is:
How should I set up these devices so when the PC is turned on it receives an address from DHCP in the 1.1.1.0/24 range and when a phone is turned on it receives an address in a 1.2.1.0/24 range?

The second part of this question is:
Once we establish how to get DHCP to properly hand out addresses to our respective devices, where do I set it up so when a voice call is made from a phone it sends the data to the sip proxy before to determine routing? Is this done in DNS?

I have read some information regarding superscopes and setting up my dhcp server with a superscope that includes the 1.1.1.x scope and the 1.2.1.x scope. After setting up the superscope I would add an ip-helper address into my router which would allow the router to forward dhcp requests to the dhcp server no matter what device is requesting the address. But, since my equipment is not on separate physical networks, I am not sure how the router will determine where each request is originating from.

If it is relevant information I have included the models for my network equipment and phones below:
Polycom soundpoint IP560 phones, Cisco 2960S switch, Cisco 2811 router.

Any help would be greatly appreciated. Please be as specific as possible in your responses. Thanks to anyone who can shed some light, in advance.


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#1
October 25, 2010 at 12:16:32
The first part of this question is:
How should I set up these devices so when the PC is turned on it receives an address from DHCP in the 1.1.1.0/24 range and when a phone is turned on it receives an address in a 1.2.1.0/24 range?

You need to ascertain if the hardware is capable of doing this. Only the documentation or the manufacturer will tell you if this is possible. I doubt you'll be able to assign TCP/IP settings to both (different) devices via DHCP and will likely find it necessary to give the computers static IP's to make this work. I had to with our VoIP setup the one time I tested the configuration you'll be using (ie: PC plugged into VoIP Phone's data port).

Are you using VLAN's?

The second part of this question is:
Once we establish how to get DHCP to properly hand out addresses to our respective devices, where do I set it up so when a voice call is made from a phone it sends the data to the sip proxy before to determine routing? Is this done in DNS?

Our VoIP setup includes a controller and all VoIP goes to it. I suspect woth your setup you would use a gateway address, not a DNS address to tell your phones where to send their data.


I'm a wee bit confused by your post. Would you mind telling me why you're subnetting a class A IP address in the second octet? What is your subnet mask for these subnets?

With regard to using a superscope, if my suspicion is correct and you have to use static IP's for the computers, then it's moot and unneeded. Again, you'll have to talk to the manufacturer, or consult the documentation, to find out whether or not a PC plugged into one of those VoIP sets can even get DHCP through the VoIP set.

It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.

***William Henley***


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#2
October 25, 2010 at 12:45:20
Just some quick points to help eliminate some confusion

DNS has nothing to do with routing
No need or reason to supernet. You don't want phones/pcs in same broadcast domain]
No need or reason for iphelper [you only have one router]

what you didn't mention which is of concern

no vlans [though I don't think phones that plug to computers can work with vlans. this is where qos comes in]
no QoS


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#3
October 25, 2010 at 13:08:44
In my switch config we do have vlan 1 set up on the 1.1.1.x subnet and vlan 2 set up as voice, but does not have allocation to a specified subnet. I am not sure on how the router is configured as this is managed by sprint, but I do know it is set up with vlan 1 as data and vlan 2 as voice.

The phone does have an option for selecting a vlan in the phone.

we do have QoS set up on our switch, and I assume it will be set up on our router as well, since I have answered QoS questions in conversations with sprint prior to this setup.

So you are saying all devices can be set up on the 1.1.1.x subnet and the router will handle deciding which traffic goes to where?


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#4
October 25, 2010 at 13:12:38
I currently have one of the phones set up and connected to the PC as described and both the PC and the phone are receiving a different address from DHCP on the 1.1.1.x subnet. I am using VLANs as described in my reply to wanderer. I know that I could set up a static IP on phones or PCs and eliminate this issue, but, I would like to have DHCP hand out the address to all devices on the network, if possible.

My subnet mask is 255.255.255.0, sprint decided that our voice vlan would use the second octet change from our original scope.

Assuming that the PC and phone are capable of both receiving an IP via DHCP, how would I set it up on my network to make that happen?


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#5
October 25, 2010 at 13:46:02
Just a little additional explanation, the link below is the only reason I have mentioned DNS in this question. I am still not certain on how the ip phone knows to send data to the sip proxy, but, as of right now I am assuming the sip proxy address will be coded into the ip phone.

http://tjscott.net/voip/settingup.v...

Thanks for taking a look at this question any information on how to get DHCP to hand out these addresses properly would be greatly appreciated.


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#6
October 25, 2010 at 15:10:24
wanderer

No need or reason to supernet. You don't want phones/pcs in same broadcast domain]

Almost got caught on that one myself. The OP said "superscope" not "supernet". I had to read it through 3 times before it sunk in to my thick head.....lol

Nabrus929

I figured you were using VLAN's. The port on your switch would have to have both VLAN's assigned to it and the PVID (primary VLAN ID) should be the VoIP VLAN.

Something like:

PVID = 1
Allowed VLAN's = 1,2

I know that I could set up a static IP on phones or PCs and eliminate this issue, but, I would like to have DHCP hand out the address to all devices on the network, if possible.

Here's the problem with trying to get DHCP to offer IP's in two different subnets to two different devices plugged into the same port on a switch.

How does DHCP differentiate between the different requests? How does the server know request A is from the VoIP set and requires an IP in subnet 1.2.1.0, and how does it know request B is from the PC and requires an IP in the 1.1.1.0 subnet. Short answer is, It doesn't and that's why both devices get assigned IP's in the 1.1.1.0 subnet.

This is why I mentioned using static IP's on the computers. You can play around until the cows come home but in the end, you'll find the simple solution is the best one and that is to have one of the two devices staticly assigned their IP settings and the other on DHCP.

I am still not certain on how the ip phone knows to send data to the sip proxy, but, as of right now I am assuming the sip proxy address will be coded into the ip phone.

Like any other network device, the VoIP set's should get a Default Gateway address assigned to them with the TCP/IP settings. This should be the address of whatever device you have that handles the VoIP traffic.


My subnet mask is 255.255.255.0, sprint decided that our voice vlan would use the second octet change from our original scope.

Well, if you're not in control of the router then you have to do whatever you're told too by whomever is. It's odd to change the second octet when your subnet mask shows you''re supposed to be subnetting in the 3'd octet.

It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.

***William Henley***


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#7
November 18, 2010 at 01:48:08
I want to ask, since I think, I have the same set-up, can a media gateway be integrated with these type of configuration? I mean with DNS, DHCP. My friend said that using a media gateway allows different devices to be interlinked particularly a phone system.

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