Secondary DHCP server on router

[larnelhight]

Is it possible to use a cisco router (2821) - as a secondary dhcp server for redundancy of a windows dhcp server?  As in utilize my windows server as a primary DHCP server for clients and if the server fails, then the router takes over this role until I resolve the windows server problem?

Thanks

- L

[Reza Sharifi]

It should work as long as the scope is not overlapping.

For example, if you are using a /24, you would assign the fist 150 ip address to the window server scope and exclude them from the router's scope. On the router side you assign 151 to 254 and exclude this range from the window side.  If windows IP is the first ip helper command, it would get ip from the window server and if the window is down it would go to the second ip helper address which is the router.

HTH

[Richard Burts]

I do not agree with this answer. In my experience when there are two ip helper-address commands the router will use both. So both the Windows server and the router would get copies of the request. And both would offer addresses. It would be up to the client to decide which one to use if it receives two responses.

HTH

Rick

[ogamanya]

Hi Richard, you are partially correct. Both servers will offer an IP and the client will accept on and inform the server that I have accepted your offer an this server will mark that IP as allocated. All well and danddy. The problem comes in that if you have 2 DHCP servers holding the same scope, you will run into problems of IP duplication becuase server 1 has no knowledge of server 2 and therefore can possibly assign a client with an IP that was issued by server 2. The way to ensure this does not happen is to configure the servers as Reza mentioned.

[Richard Burts]

I agree with you that the description by Reza of the problem of duplicate scopes was quite correct. What I did not agree with was this statement:

If windows IP is the first ip helper command, it would get ip from the  window server and if the window is down it would go to the second ip  helper address which is the router.

If the distinction was not clear in my post then I appreciate your clarification.

HTH

Rick

[ogamanya]

Hi Richard,

The fault was on my side.... Reading this again I realized I had missed the distinction you made on Reza's post. My apologies

Onasis

[Richard Burts]

Onasis

Not a problem at all. It is this kind of discussion, questioning, and clarification that makes these forums such a valuable resource. Please continue to read, and to learn, and to discuss, and to post in these forums.

HTH

Rick

[larnelhight]

So does this mean that utilizing a router for seconday DHCP server is not possible?

[Karsten Iwen]

As Windows normally prefers the first DHCP-answer, it could work if you make sure that the IOS-DHCP always responds much slower then the Windows-Server. But thats not so easy to achieve.

[Richard Burts]

I do not want to be overly picky in my response. But I believe that we need to be careful how we understand the meaning of having a secondary server.

One way of understanding the secondary server is that we want to eliminate a single point of failure. In this case we do not want to rely on having just the Windows DHCP server since it is a single point of failure. So we could configure the router to also do DHCP (being careful that we do not duplicate the addresses in the DHCP pools). In this sense it is quite possible to have a router as a secondary server.

But the original poster was quite clear in describing the concept of secondary server to be that as long as it is working the Windows server would be the only active server and that the secondary server should function only when the primary is not functioning. In that sense we are not able to do this on a router.

HTH

Rick