Redundant DHCP servers - How?

[Svend]

Is it possible to have redundant DHCP servers?

Some arrangement with dynamic failover would be nice.
I can imagine having two servers configured using scopes
which are different subsets of the same subnet. But in my
mind this would require manual maintenance of scope
properties as well as manual fail over.

Svend.

 

[Matjaz Ladava [MVP]]

One method would be cluster, but this is too expensive. another way is 20/80
method, where you assign 80% of you scope on one DHCP server and 20% on
another DHCP server (and exclude appropriate range on each DHCP server), but
you can always have a spare DHCP server (offline) ready on your network and
connect it in case of DHCP server failure. If you leasing time is enough
long (>1 day), you can rebuild DHCP server in this time.

--
Regards

Matjaz Ladava, MCSE, MCSA, MCT, MVP
Microsoft MVP - Active Directory
(e-mail address removed), (e-mail address removed)
http://ladava.com

 

[Stuart]

Hi
Keepin a spare won't really work as when you bring it on
line it will try to assign IP addresses that are already
assigned.
Rebuilding the DHCP server will also not really work for
the same reason as above and also machines that were
assigned an address 22 hours previously (assuming 1 day
lease) will need to renew their lease in two hours.
The only real way is to use the 80/20 method. If you have
more than one subnet then use the the 80/20 rule. If you
use only one subnet then use 50/50.
DHCP A
IP address range 192.168.0.1 to 192.168.0.254
exclude 192.168.0.128 to 192.168.0.254
(this will lease 1 to 127)

DHCP B
IP address range 192.168.0.1 to 192.168.0.254
exclude 192.168.0.1 to 192.168.0.127
(this will lease 128 to 254)

Make sure you use a subnet that has at least twice as
many addresses as you have hosts (and any future growth).
This way if one DHCP goes down the other can handle all
the addresses.

Stuart

 

[Svend]

Thanks for the input.

I take it then that I would need to maintain any server
options (Default Gateway, Reservations, DNS servers etc.)
manualy on both servers - there is no way of sharing the
database for these items?

Also in our case we use the routers on remote sites to
relay DHCP requests back to the central DHCP server. I
would need to address this issue - they are cisco 1700
series routers, not sure if you can have multiple "ip
helpers" on a cisco router. Any idea?

Thanks,
Svend.