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Mike Clites
Beginner

Configure Cisco Router with ip-helper for redundant/failover MS DHCP servers

I am trying to figure out how to configure a router to point towards a pair of off Lan MS dhcp servers in an 80/20 redundant/failover configuration.

I typically use the ip helper-address x.x.x.x under the subinterface of the router for a single off Lan dhcp server.

Now we want to make the dhcp servers more resiliant. Now I have 2 dhcp server ip addy to point towards BUT we want it with a time delay on the 2nd server so the 1st (primary) server gets most if not all the dhcp requests and the 2nd (backup) dhcp server is basically waiting for a failover.

Any help would be appreciated.

This is how it sits now (1 off Lan dhcp server only)

interface GigabitEthernet1/0.133

description My Data

encapsulation dot1Q 133 native

ip address 172.19.133.161 255.255.255.240

ip helper-address 172.16.1.175

end

I have a new secondary MS dhcp server ip addy  172.16.1.176

1 ACCEPTED SOLUTION

Accepted Solutions

Hi Mike,

I think the solution is here:

http://blogs.technet.com/b/teamdhcp/archive/2009/01/22/how-to-configure-split-scope-using-wizard.aspx

They talk about delaying the DHCP offer on the secondary server.

Regards.

Alain.

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View solution in original post

8 REPLIES 8
cadet alain
Mentor

Hi,

if you put the 2 ip helper-addresses then first one to answer will give out the address.

Maybe you should investigate if there could be a MS solution to do what you want.

Could you show the topology position of the 2 servers from the client standpoint.

Alain.

Don't forget to rate helpful posts.

Cadet,

Thank you for taking the time to respond.

You are correct if you put 2 helper statements in then they act in a "load sharing" capacity.

We do not want load sharing we want failover.

server #1 will always or almost always be available to give out the addy's. That is why the 80/20 configuration suggested by MS.

server #2 will only respond when server #1 is down or super busy.

What I need is a way for me to configure the cisco to make it offer the request to both servers but with something like a 1 second delay to the 2nd server.

Yes I was just speaking to MS guy and he said he does not know specific commands and also MS frowns upon them assisting with configuration of non MS products. Either way he was not going to offer more than the standard "here is MS best practice."

Site A) clients

Site B) primary and backup MS DHCP servers.

DHCP is done across the T1 Wan.

Again, I have this working already in several locations BUT not with dhcp failover.. Now we want dhcp failover

Hi,

taken from Microsoft technet:

By default, the DHCP service does not perform any conflict detection. To  enable conflict detection, increase the number of ping attempts that  the DHCP service performs for each address before leasing that address  to a client. Note that for each additional conflict detection attempt  that the DHCP service performs, additional seconds are added to the time  needed to negotiate leases for DHCP clients.

This might something to investigate.

Alain.

Don't forget to rate helpful posts.

Cadet,

Nice snippet from technet, thanks.

But no it does not apply to having one server as the primary and one as the backup from the Cisco side.

Also, even if you do increase the ping time on the dhcp servers and run them in a load sharing (which is what that article was about) you could still run into the issue of an address already being assigned by dhcp server A but the device being offline. So dhcp server B says hey I cant ping x.x.x.x so it must be available since I dont have it assigned in my pool. So dhcp server B assigns x.x.x.x then the device that was already assigned that addy from dhcp server B (lets say 4 hrs ago) comes online.... Now you have an IP address conflict and 2 dhcp servers thinking they have a valid pool.

No thanks.

Jon Marshall
VIP Community Legend

Mike

Is there any specific reason you do not want to use both DHCP servers at the same time ?

Jon

Jon,

I am the foot soldier.

The server guys talk to MS.

MS says "best practice for dhcp redundancy is 80/20."

This gets relayed back to our common boss who likes the idea.

Mike is then instructed by said boss to figure it out.

Problem is, I cant seem to find anyone or any article stating that Cisco actually can allow 2 or more ip helper addy's in a delayed request configuration.

Again,

We dont want load sharing, we want Failover/redundancy in dhcp servers that area off net aka.. off site thus the need to have the router use some form of ip helper statement.

You would think it would be rather simple but from my lack of article finding and no clear discussion of this topic here in the forums I am begining to think I have to open a configuration assistance case with the TAC.

I do appreciate your guys feedback and questions.

Mike

Hi Mike,

I think the solution is here:

http://blogs.technet.com/b/teamdhcp/archive/2009/01/22/how-to-configure-split-scope-using-wizard.aspx

They talk about delaying the DHCP offer on the secondary server.

Regards.

Alain.

Don't forget to rate helpful posts.

View solution in original post

Alain,

Thanks for the link.

That looks like a great article which I will pass along to the server team once I get with the Cisco TAC.

I have already openned a case.

Since I as the network guy was tasked to try to find a "Cisco" side solution I am going to do my diligence before going back to them and telling them that the best or only solution I can come up with is for the dhcp servers to be configured with a delay in the secondary.

They were specifically told by the MS rep that we should be implimenting it at on the sending side; in this case that is the Cisco.

If after I speak with the TAC we conclude there is not a viable option for this delay function then I can go confidently back to my boss and the server team and explain our options.

*update*

I have a case open with Cisco and the TAC engineer says there is no way of delaying the send of multiple "ip helper-address" servers.

He also suggested that the dhcp reply be delayed on the server.

This is supported on windows 2008 servers.

Now that I have confirmed this with the TAC I can go back to my team and explain where the delay needs to be inserted.

Thanks!