Back Up Windows DHCP Server 3

[NetworkGuy101]

Is it possible to have a back up DHCP server solution for Windows. Like if one server goes down one takes over or run 2 that sync databases?

 

[GrnEyedLdy]

You can run two DHCP servers on the same subnet as long as they are configured with different scopes, (that don't overlap).

For example, if your internal network address is;

192.168.1.X/24

and you want to hand out addresses in the range of;

192.168.1.20 thru 192.168.1.200

You simply need to break this range down into 2 different scopes and define one on each server.

I believe a 70/30 is what I have heard recommended. So with 180 addresses in the above example you can make your first scope 192.168.1.20 thru 192.168.1.146 and on your second DHCP server, set the scope to 192.168.1.147 thru 192.168.1.200


Hope this helps,


Patty

 

[NetworkGuy101]

Well That would be fine for about a backup type solution. The major issues is that My DHCP Server went down sometime late Sunday Night/Early Monday Morning. It brought down most of the company because there was no DHCP Service running. I was hoping for 2 DHCP Server that shared the same scope and just would sync with each other to keep the integrity of the scope.

 

[GrnEyedLdy]

Maybe you misunderstood me, although your two DHCP servers cannot SHARE the same scope, you can take your original scope and break it into 2 scopes. These 2 scope must each be unique and cannot overlap addresses, but they are still on the same network.

Then either DHCP server who hears the Broadcast request for an IP address will respond. Your client machines will select the first IP address offered, regardless of which DHCP server made the offer. Remember, they are both on the same Subnet and both handing out addresses that will work on that subnet.

If DHCP server 1 is down, DHCP server 2 will still be able to respond to requests and visa versa. When both machines are up and working, there are simply two DHCP servers available to hand out adddresses. When one DHCP server is down, the other still functions.

DHCP servers do not have the ability to coordinate with each other in order to find out which addresses each server has handed out.


I hope this makes more sense. Sorry if my post was confusing.


Patty

 

[NetworkGuy101]

No I understood you perfectly and I think it is a good idea. I was just hoping that there was a way to use the same scope with 2 servers without having address conflicts i.e. the address pools staying in sync. I guess it will work for a backup solution for short periods.

 

[GlenJohnson]

Patty, is the 80/20 rule, and a rough explanation can be found at my faq.
faq950-4927

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[NetworkGuy101]

Your FAQ is good but how can a host get a IP from and Exclusion Range? Exclusion Ranges are ranges to be EXCLUDED from the Pool of available IPs. Im not trying to be smart but if there is a way to do this I would like to know.

 

[Castor66]

Here's how it works:

PC Starts up and shouts "I NEED AN IP ADDRESS"

DHCP server #1 hears it and says "Take 192.168.1.40"

The second DHCP server also offers an address but the PC replies "I've already got one, thank you very much!"

The second DHCP server sulks.

DHCP server #1 smirks!

DHCP server #1 crashes!

PC Starts up and shouts "I NEED AN IP ADDRESS"

DHCP server #2 hears it and says "Take 192.168.1.201"

PC says "Thanks!

DHCP server #2 smirks and flips the bird at DHCP server #1!

Get the picture?

 

[GrnEyedLdy]

I love that explanation!

;-)


Patty

 

[NetworkGuy101]

Yes I understand how DHCP works but that is not what is in the FAQs. Possibly a typo.

 

[MTGNet]

If you change your lease time to as an example 14 days. You would have that long to get a new DHCP server up and running before it affected any of your users.

 

[GlenJohnson]

Ok, gang, now is the perfect time to see if we can perfect the 80/20 faq. I've had a couple of people ask for more details. Any suggestions on how we can clarify it, I'm willling to listen. All suggestions that are used, the person will be mentioned in the faq and thanked for there help officially. Thanks. (Oh, I don't think management would like servers flipping off each other in the faq, but I too liked it.)

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[GlenJohnson]

Oh, I almost forgot. The 20% on each server are excluded for use on it's own. 80% are used for server a, with 20% being excluded from use, so if server b needs addresses, they will be drawn from the 20% that server a cannot use. The same goes vica versa. 20% that server be can't use, is left alone so server a can use it if needed. Each server is set up with 80% it can use, and 20% it can't use so it's available for the other server. OK?

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[GlenJohnson]

Think before you speak. From MS

DHCP Server 2 is configured to lease most (about 80 percent) of the available addresses. DHCP Server 1 is configured to lease the remaining addresses (about 20 percent).

This scenario allows the local DHCP server (DHCP Server 2) to respond to requests from local DHCP clients most of the time. The remote or backup DHCP server (DHCP Server 1) assigns addresses to clients on the other subnet only when the local server is not available or is out of addresses. This same rule can be used in a multiple-subnet scenario to ensure the availability of a DHCP server when a client requests a lease.

http://www.microsoft.com/resources/...00/server/reskit/en-us/cnet/cncb_dhc_ogjw.asp

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