Redundant DHCP Server
There is no setting to make DHCP redundant, but you can build it so that
there is a redundant configuration. Clustering is not recommended as it can
run into problems on DCs/ DNS/ DHCP and there are enough configuration
options built into the product to make that not needed.
It will be tempting to build each DHCP server with half the scope but resist
this. DHCP works by accepting whatever server responds first to the server
location broadcast. If each has half of the DHCP range and it tries to renew
with the wrong server, the workstation will get a NAK message.
Unfortunately, the workstation may not always try again when it gets the NAK
and will simply drop off the network. I like to call this NAK poisoning
(though I am sure this is probably not my term)
To get around this, have both servers host the whole range and have
reciprocal exclusions so if the renewal is out of range, the server will
offer a substitute address rather than a NAK.
Example on a 192.168.1.0/24 network
Server 1: 192.168.1.0/24 range excludes 192.168.1.129-254
Server 2. 192.168.1.0/24 range excludes 192.168.1.1-128
Note that this is 50/50... Note that normal distribution is 80/20 here the
80 can handle the entire required number of hosts. Just remember if you're
going to split the servers across a routed boundary to include an IP
Helper-Address (on Cisco)command to allow foreign solicitations. The local
one will almost always respond first unless it is down.
Hope this helps.
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