DHCP and the 80/20 rule

[Techhead]

What is so significant about the 80/20 rule for DHCP? Microsoft
recommends you host 80% of a zone on one DHCP server and the other 20%
on a different server. Why not 50/50? 50/50 seems more logical to me...

If anybody knows what the realm of this 80/20 rule is, please let me
know.

Thanks,
Brian

 

[Bob I]

http://www.google.com/search?hl=en&q=80/20+rule+for+DHCP

 

[Techhead]

It appears that this rule only applies to multiple subnets but what
about running 2 DHCP servers on the same subnet?

 

[Bob I]

No can do.

 

[Richard G. Harper]

Sure you can. Run both in the same subnet, use excisions on each server to
exclude the range of addresses the other is giving out. Configure both for,
say, 192.168.1.x addressing; then on one server exclude the addresses
192.168.1.200-255 and on the other exclude the addresses 192.168.1.1-199 so
they don't step on each other's toes.

I don't consider this wise unless you're running a really big network (in
which case you probably have multiple ranges anyway) but it surely can be
done.

--
Richard G. Harper [MVP Shell/User] (e-mail address removed)
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[Bob I]

Wouldn't that cause a problem with the system changing the IP address of
a clients at renew time?

 

[Andrew Story]

One live dhcp server per subnet. Is it not?

Wouldn't that cause a problem with the system changing the IP address of
a clients at renew time?

 

[Bob I]

That was my belief. (let's say recommendation )

One live dhcp server per subnet. Is it not?

 

[Techhead]

Yes, but it's not going to give out an address thats excluded in the
zone and is offered by a DIFFERENT dhcp server for the SAME zone. You
can have multiple DHCP servers running on 1 subnet. Trust me, I'm doing
it right now with no problems. I'm just wondering about the 80/20 rule.

 

[Techhead]

This is what happens when you have more than on DHCP server on a single
subnet. Which ever DHCP server responds first to the client request,
serve's the request. When a client renews a lease, it tries to renew
from the last DHCP server it received an IP from, otherwise it sends a
broadcast and whichever server responds first again, serve's the lease.
If the first DHCP server in my subnet as leased out all of it's IP's,
then the second DHCP server will only respond to client requests.

As long as the 1st DHCP server does not assign the same addresses as
the 2nd server, you are fine.

 

[Richard G. Harper]

Why would it? I don't care what IP address my computer has.

Mind you, I said I don't recommend it - just that it can be done.

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[Richard G. Harper]

It's a recommendation, like any other. If you don't like it and/or it
doesn't meet your needs, consider something different. The idea is to get
the majority of all clients on a single server.

--
Richard G. Harper [MVP Shell/User] (e-mail address removed)
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[Techhead]

Well I am still not seeing any hard evidence to backup why you would
want the majority of your clients on one DHCP server. I don't see any
logic behind the idea and knowing that this 80/20 recommendation was
derived from Microsoft, I'd have to think twice any ways. This is still
up for debate and anybody who implements a DHCP rule other than 80/20
with real world results, I would like to here what you have to say.
Until I understand the significance of the 80/20 rule, if any, I am
still stuck on the more logical 50/50.

 

[Guest]

This is taken from
http://www.windowsdevcenter.com/pub/a/windows/2004/04/13/DHCP_Server.html

The 80/20 rule:

"...The idea behind splitting the addresses 80/20 is because the 80 percent
of addresses is normally sufficient for all the addresses needed on a subnet,
and DHCP leases are typically three days, so if your subnet's main DHCP
server goes down for a few hours then it's unlikely that more than 20 percent
of the machines on that subnet will need to renew their addresses during the
downtime, making the 20 percent pool of addresses sufficient."

And now for the logical 50/50 rule:

"...Because of considerations like these, many network administrators prefer
a more conservative 50/50 rule, which actually works even better in a single
subnet environment anyway, provided both servers have enough addresses scoped
to cover the needs of all the clients on the network. Then if one server goes
down, the other one can take up the slack for as long as it takes to get the
first one up and running again."

Hope this help

SethDK

 

[Guest]

Where I work there are two identicle domain controlers. Both handle AD,
DHCP, DNS, WINS. One class C address range and one subnet, about 125-150
users. I believe the idea is that if one DC fails the other will take over.
There is no 80/20, 50/50. They are identicle. This is working fine.

Is there something flawed here?

 

[Richard G. Harper]

Yep. You're lucky you haven't had a mess of "IP Address Conflict" problems
with this setup. There should be only one DHCP server per address range or
else you risk conflicts.

--
Richard G. Harper [MVP Shell/User] (e-mail address removed)
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* for the benefit of all. Private mail is usually not replied to.
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[npsingh123]

I understand that it is a six year thread.

Still I need to post because people are quoting Mich Tulloch to suit themselves.

==============================================================
Here's another interesting approach: configure both DHCP servers on your network with identical scopes. Now, normally this would cause problems because you might end up having the same address leased by both servers, causing clients to hiccup. However, if you enable address conflict detection on your DHCP servers and set it to 2, then each DHCP server will test an address three times to make sure it's not already being used before leasing it to a client, and this will prevent one server from leasing an address that the other server has already leased


by Mitch Tulloch, author of Windows Server Hacks
04/13/2004



http://windowsdevcenter.com/pub/a/windows/2004/04/13/DHCP_Server.html

There a screenshot on that page to clarify things.

===========================================================
- Nagendra Pratap Singh, MCSE, MCITP, VCP