VoIP Design Question

I know an accurate answer will likely depend on which vendors/models/versions I'm using, but I don't have all that info right now and I guess I'm fishing for a more generic, theoretical type of answer.

We have two physical buildings at our location. Right now the buildings each have their own network. Bldg1, for example, is Bldg2 is The two buildings are connected via ppp T1 circuit. Each building has its own router. The buildings can talk to each other just fine. All the servers are in bldg1. Bldg2 even gets its DHCP addresses from a server in bldg1 using ip-helper on the routers.

We are considering going to voip phone system. The only time I've touched voip before, the vendor told us to put voice in one vlan, data in another. Is this just best practice, or is it a requirement? Because I'm seeing a potential problem in the fact that each building is on its own network. Even if we create two vlans in each building and call them the same name -- they are still going to be completely different vlans in completely different subnets, right? Are we going to be able to have only one phone server in bldg1 that will also provide service to the phones in bldg2? Can you "route vlan traffic" across a WAN link like this?

The fact that we can hand out DHCP across a WAN link is encouraging, but I just don't know how voice traffic will be affected.

Hope some of this makes sense. Thanks.

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Create two different voice vlans, one in each building. Generally, and at least when you are using call manager and other enterprise solutions, the phones will boot and request an address via dhcp. The dhcp server will hand out an IP, and you'll need to populate the tftp server address in the optional dhcp field. This will tell the phone where to get its image and configuration once it accepts its IP address. This should negate the need of having a single voice vlan across two sites, its perfectly legitimate to have two voice vlans, and a single voip server (or pair).

To answer your other question, nothing is technically wrong with sharing voice and data, its just bad practice. Voice should always be prioritized over data, and if they are in separate vlans, then data traffic (like ftp, broadcast, etc) will be kept separate. Once you hit a WAN link, you'll have to prioritize voice, but since its a separate vlan, its easier to mark the traffic at layer 2 or layer 3. Once its marked, you can setup your QoS policy to prefer one bucket over the other. Therefore, you should really keep your voice and data vlans separate.

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Even though it's possible to run a phones and computers in the same segment, it's not recommended. PCs can generate broadcasts and multicasts, which may screw up your voice traffic. So, it's better to isolate them.

In addition to creating a different VLANs on the switches, you will need to create a corresponding IP interfaces. You have not stated, what type of switches and routers do you have. If you have Layer 3 switches, then you just create a VLAN interface and assign an IP address to it (for example, for the Bldg1, and for the Bldg2). If you have Layer2 switches, and IP addresses configured on the routers, then you must configure port to the router as a trunk, and configure subinterfaces on the router.

If you serve DHCP on your Voice server (for example, Cisco CallManager), then you place the server in the segment, and configure "ip-helper" on the segment.

Your biggest concern should be the Quality of Service (QoS) for the link between servers. First of all, you should configure a low-band codecs between locations (for example, G729). It will minimize a bandwidth requirements for each call (it will be 20 kBit versus normal 80 kBit for G.711). But even with all that you must configure your voice traffic with high priority. It's not that difficult, but requires some time to find proper example at Cisco's website.

Other than that - everything should work fine. Depending on your budget, you should look for a backup solution in case your WAN link goes down. Ideally you may need to install a dedicated router and circuit for voice, however it may be not reasonable. Also you may need to install a voice card at your router with POTS lines, and configure it for SRST (Survivability). Without that if your link goes down, your IP phones will be useless.

Good luck,

Mike CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc. CCIE R&S (in progress), CCIE Voice (in progress)

------ Headset Adapters for Cisco IP Phones

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