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 192.168.10.0/24. Bldg2 is 192.168.100.0/24. 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.