Building a network with DHCP and Voice

[Islam Nadim]

Hello All,

     I'm in the process of building a network that will consist of the below:

2 Routers (Primary and Backup)

2 Switches (16-Ports each)

20 IP Phones (10 on each Switch)

Voice Gateway (Don't know anything about it)

Wireless Access Point (Still not Confirmed)

Here is what I'm looking to achieve ..

First:

The routers will have HSRP running between them to ensure connectivity if the primary link fails.

First Question: I want the primary router to act as a DHCP Server .. The Scopes on it should be as follows:

• 192.168.1.0/24 -> The Main Network
• 192.168.1.1-192.168.1.5 -> Excluded
• First Scope: 192.168.1.6-192.168.1.29 -> PCs
• Second Scope: 192.168.1.30-192.168.1.50 -> IP Phones

Second Question: Can I make the backup router act as a backup DHCP Scope as well?

Third Question: Regarding the Voice Gateway, what are the options I have and which is the best (Stability wise)?

Forth Question: For the default route 0.0.0.0, should I make the next hop the Virtual IP of the HSRP?? ip route 0.0.0.0 0.0.0.0 192.168.1.1

I believe thats all and hope I find answers as well as gaining experience from you

[Richard Burts]

We do not have enough information to answer all of your questions. But here are answers to the ones I think we understand and have enough information to be able to deal with.

first question is not entirely clear. Will the IP phones be in one VLAN/one subnet and PCs in another vlan/subnet or do you plan for them all to be in the same subnet/same vlan?

second question. it is possible to have the backup router act as backup DHCP server but it is more complicated that you probably realize. You can not just configure the same scope on the backup router. If the backup router is going to act as backup for DHCP then it needs it own scope. Otherwise it will be assigning the same addresses as the primary and you will get address conflicts in the network.

third question. we need to know much more about what you are planning for the voice gateway to be able to advise on this.

fourth question. no you should not have the default route point to the HSRP address. The default route should point its next hop to some device outside the router to which it will send traffic going to destinations that are not locally connected on the router. Pointing the default route to the routers own address defeats this process.

HTH

Rick

[Islam Nadim]

Hello Rick,

     Thanks for the fast response

Regarding the first question: I'm not sure if it is better to have them in the same subnet or not .. But I'm thinking different subnets.

Regarding the second question: What I understood, I can let the primary router lease 192.168.1.0/24 and the backup 192.168.2.0/24 (just an example)

Regarding the third question: As I stated before, I know nothing about the Voice .. But I need each IP Phone to have a 4-digit extension .. And there will be 2 separate lines that will be used for external calls .. Really don't know how can this be achieved.

Regarding the forth question: So, since I'll be having dynamic WAN IPs, it should point to the WAN interface, right?

Best Regards.

Islam.

[Richard Burts]

Islam

My experience is that it is more common to have phones in one vlan/subnet and PCs in a second vlan/subnet. It can work either way and largely depends on what the phones expect. Do you know what the phones will expect and do they support a voice vlan and a data vlan?

Yes you could have primary router lease 192.168.1.0/24 and the backup lease 192.168.2.0/24. Different networks on a router usually means different interfaces. So this might be more of load sharing than of true backup. If you want the two routers to operate as true backup for DHCP you might do something like this

- configure the router interfaces to do HSRP with interfaces selected from 192.168.1.0/24

- configure a DHCP network scope on the primary router of 192.168.1.0 through 192.168.1.127.

- configure a DHCP network scope on the backup router of 192.168.1.128 through 192.168.1.255.

The voice gateway is a piece of equipment. If these are Cisco IP phones then the voice gateway is frequently configured on another router in the network. Who is designing the phone/voice part of the network? They should be able to tell you what they plan for the voice gateway.

If you have dynamic WAN you should be able to relect that in your default route. Sometimes it would look something like ip route 0.0.0.0 0.0.0.0 dhcp.

HTH

Rick

[Reza Sharifi]

Islam,

I agree with Rick.  If your switches are POE capable and you want to connect your PCs to the back of the phones, then you want to put the voice traffic in one vlan/subnet and the data traffic in another vlan.  The reason for that is that most of the time you apply QOS to voice traffic and not data and so if you have them in separate vlans, you can easily apply QOS to the voice vlan/subnet.

Regarding the gateway, I am no voice expert, but as far as I know the voice gateway enables you to have VOIP in your LAN and PSTN on the WAN.  For example: when your users in the office call each other, they use VOIP, but if they need to call home, they use PSTN and that is the gateways job to provide call routing between internal users and PSTN.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/srnd/design/guide/gatewy.html

HTH