I have 3 sites each with a domain controller and a member server in a domain. Each domain controller is a dhcp and dns server as well. This weekend, one of the DC's wasn't functioning and users could not login to the domain I believe since they could not get a dhcp address. Is there a way that I can have another server be a backup for dhcp and dns so that if the primary DC goes down, another server could step in and give out ip addresses and dns requests. thanks in advance.

You could use DHCP forwarding in your network. As clients accept DHCP replies from the first server that replies to them they would commonly accept the answer of the local DHCP server. You might need to create multiple DHCP scopes as your sites might have different IP subnets. I can't find a webpage describing this scenario, however, I know for sure that it is mentioned in the MCITP 70-642 network infrastructure book from Microsoft press.

First, Check the configured lease times in your scopes. By default Microsoft sets these to be 8 days, which should allow a DHCP client to continue working even when the DHCP server is unavailable. If that time < less than 5 days then i would consider increasing this to 8. By design DHCP clients should renew their lease no later than when 50% of the lease has expired. So with an 8 day lease you server could be down 3-4 days (friday,sat,sun,mon) before an issue would arise with the clients that already have leases. New clients are always affected when no dhcp server is avavilable.Secondly, you need to understand your WAN and its risks of failure to properly determine where to placeand configure DHCP. For example if you have a very reliable WAN provider using MPLS and backup connections you should centralize your DHCP management to your main office by configuring BOOTP forwarders on the routers. If you have a moderately reliable WAN you can do the same, but make sure your DCHP leases are significately long (item 1 above) to accomodate a WAN outage. If you have a site to site VPN or something else considered un-reliable you should stick with the de-centralized approach of placing DHCP servers close to the clients.Thridly, configure split scopes or some other fault tolerance solution for DHCP. While a lot of emphasis is placed on split scopes across two DHCP servers, that is difficult to manage. I prefer scripting the database backup and store in a separate location/backup. I can re-build a DHCP server quickly with the same IP with less management overhead then maintaining multiple DHCP servers.

Hello,configure in each site a second server, each with the half of the scope according to:http://technet.microsoft.com/en-us/library/cc780311(WS.10).aspxBest regards Meinolf Weber Disclaimer: This posting is provided "AS IS" with no warranties, and confers no rights.