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q: setting up dhcp server as failover i have 2 domain controllers running at my company;
dc1 has the following roles;
adds
dns
dhcp scope - 192.168.2.100 - 192.168.2.240
dc2 has the following roles;
adds - replicated
dns - secondary dns
i also have 2 pfsense gateways both have a different isp connection, dhcp relay is not enabled on any of the machines. 
can anyone suggest a way for me to install dhcp server as failover on dc2 please, should i use split scope etc... 
help is much appreciated :) thanks. 

a: i think you're on the right track with what you want to do here. you haven't specified which version of windows server you are running so i am going to assume windows server 2008 r2 for the purpose of my response.
the first step would be to install the dhcp role on your 2nd domain controller. there is a good walkthrough with the steps necessary to do this at howtogeek but the basic steps are:
open "server manager" > right-click *"roles"** > select "add roles"
add the "dhcp server" role listed under "server roles" and then you'll need to configure the settings, which brings us to the next part of your question. how to beset setup multiple dhcp servers in a failover configuration. 
using a split scope is an option for this, but it is not the only option. in fact, you really have three options:

1.split scopes - splitting your dhcp scope between multiple servers
advantages: 


*

*cheaper than other solutions as it does not require the purchase of an external disk system for clustering

*you're protected from database corruption because if either of your dhcp servers becomes unavailable due to a db corruption, the other one can step right in and server dhcp while the down system is being repaired


disadvantages


*

*in the event of an outage of one of your dhcp servers (whether due to system upgrades or an unplanned outage), you'll be limited to half of the dhcp scope (assuming you do a 50/50 split). if ip availability is not a concern, this may not be a big deal.



2.dhcp clusters - multiple servers acting together to server ips in the same dhcp scope
advantages


*

*easily administered from the cluster administrator mmc

*systems can be upgraded while online

*if a single server in the dhcp cluster is offline, 100% of the dhcp scope is still available


disadvantages


*

*if the clustered dhcp database becomes corrupted, the dhcp service will be unusable until repaired (translation: single point of failure)



3.dhcp standby server - in this configuration, your secondary dhcp server would be configured exactly like the primary, but you would leave the service disabled until it was needed due to an outage of the primary system
advantages


*

*you get the full dhcp scope availability that the clustered solution offers while still keeping the minimal hardware costs that are associated with the split scope definition


disadvantages


*

*manual intervention is required to start the service in the event of an outage of your primary dhcp server

*you will run into dns record ownership issues if dhcp is configured to update dns records due to them being own by the failed dhcp server

*extra network bandwidth is required when first bringing the secondary server online due to it having to rescan the network to assign ips (if you go this route, i highly recommend enabling conflict detection on your secondary dhcp server)



recommendation: my recommendation for you would be to go with the split scope solution you've already been leaning toward, as long as you have enough available ips that half of the total dhcp scope would be sufficient for your needs (so as not to run into any problems if one of the servers is offline).

directions: configuring split scopes


*

*open the dhcp microsoft management console (mmc) snap-in.

*in the console tree, double-click the dhcp server you want to configure.

*right-click ipv4 and then right-click the scope you want to split.

*click advanced and then click split-scope.

*follow the instructions in the dhcp split-scope configuration wizard.


a: you don't give any information regarding what os is running on dc1 or dc2, but in short: yes you should install dhcp on dc2 and split your dhcp scope between dc1 and dc2. if both dc's are running w2k8r2 then the dhcp management console has a split scope wizard that you can run that will do the work of creating a split scope for you.

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