My company has numerous physical offices (for purposes of this discussion, 15 buildings). Some of them are well-connected to our primary data center via fiber. Others will be connected to the data center by P2P T1. We are in the beginning stages of implementing an Avaya VOIP telephone system, and we will be replacing a significant portion of our network infrastructure in the process.

In tandem with the phone system implementation, we are going to be re-addressing some of our networks, and consolidating most of our Windows domains into one (not all domains, just most). We currently have quite a few Windows domains, and they of course each have their own DNS zones. A few of those networks currently use DHCP, but the majority use static IP assignments for every device. I'm tired of managing static assignments -- I want to use DHCP configuration on everything except servers. Printers and etc will have DHCP reservations. The new IP phones will need to get IP addresses from DHCP, though they need to be in a separate VLAN from the computers/printers/etc.

The computers and printers need to be registered in DNS. That's currently handled by the Windows DHCP servers on each of the respective domains. We need to place a priority on DHCP and DNS being available on a per-site basis (in case something were to interrupt the WAN connection) for computers and (primarily) phones. Smaller locations (which will have IP phones but not be a member of any Windows domain) will not have any Windows DNS/DHCP server(s) available. We also are looking for the easiest way to replace a part if it were to fail. That is to say, if a server/appliance/router hosting DHCP were to crash hard, and we couldn't extremely quickly recover the DHCP reservations and leases (and subsequently restore them onto a cold spare), we anticipate that bad things could happen.

What is the best idea for how to re-implement DNS and DHCP keeping all of the above in mind? Some thoughts that have been raised (by myself or my coworkers):

• Use Windows DNS and DHCP servers, where they exist, and use IP helpers to route DHCP requests to some other Windows server if necessary. May not be acceptable if the WAN goes down and clients don't get a DHCP response.
• Use Windows DNS (everywhere, over WAN in some cases) and a mix of Windows DHCP and DHCP provided by Cisco routers. Every site would be covered for DHCP, but from what I've read, Cisco routers can't handle dynamic registration of DHCP clients to Windows DNS servers, which might create a problem where Cisco routers are used for DHCP.
• Use Windows DNS (everywhere, over WAN in some cases) and a mix of Windows DHCP and DHCP provided by some service running on an extremely low-price linux server. Is there any such software that would allow DHCP leases granted by these linux boxes to be dynamically registered on the Windows DNS servers?
• Come up with a Linux solution for both DNS and DHCP, and deploy low-price linux servers to every site. Requirements would be that the DNS zone be multi-master (like Windows DNS integrated with Active Directory), that DHCP be able to make dynamic DNS registrations in that zone, for every lease (where a hostname is provided and is thus possible), and that multiple servers be either authoritative for the same DHCP scope or at least receiving a real-time copy / replication / sync of the leases table so that if one server dies, we still know which MAC has what address.
• Purchase dedicated DNS/DHCP appliances, deploying to all sites. From what I read/see, this solves all of our technical problems. Then come the financial problems... I don't have a ton of money to spend on this.
• Or, some other solution that we've thus far overlooked and will consider upon recommendation.

Can Cisco routers or Windows servers sync DHCP lease tables so that multiple servers can be authoritative (or active/passive for all I care) for the same scope, in case one of the partners were to fail? I've read online (repeatedly) that ISC's DHCP is able to maintain the same lease table across multiple servers, in order to solve this problem. Does anyone have any experience or advice to regarding that?