DHCP with multiple IP networks

[OCCSteve]

We have 2 ip networks with separate gateways on a single router interface. One was existing with DHCP services provided by a Windows 2003 Server. The new network works fine with static IPs, but when we add it's scope to the DHCP server we cannot get an address.
The NIC on the server has 2 addresses-1 from each network. We can ping each address fine. Does anyone know why when we turn off the original scope, the server will not distribute an address from the new scope?

DHCP experts are apparently hard to come by.

Thanks.

 

[cajuntank]

In some form or fasion, you will need to implement DHCP relay to forward DHCP to the other subnet.

Personally, instead of using your server as a router, it sounds like you might already have a core router for your subnets, so just do a search on "ip helper-address" on google. This line only needs to be added to the other subnet sub-interface on the router. The ip helper command will reference the IP address of the DHCP server and your good to go.

Doing it this way, you can simplify your addressing on your server down to just the 1 ip address and not take up needless resources by your server doing duty for what your router already does.

 

[OCCSteve]

I'm familiar with helper-addresses, but I've only used them across vlans or WANs. To clarify, the gateway addresses are separate secondary addresses on a single router interface, with no vlans--xxx.xxx.33.1(existing), xxx.xxx.36.1(new). I think your saying I can apply the ip-helper-address(33.2)to only the 36.1 secondary. I wasn't sure you could do that.

So am I wrong in thinking?: The server will first use up the available pool from the existing scope. After that, requests will be denied so they will end up at the router, at which point the 36.1 gateway will forward them back to 33.2 to get an address from the new scope(with the 36.1 gateway).

If that sounds right, please let me know. And yes, I see the wording, I am often wrong in thinking!

Thanks.

 

[cajuntank]

Oh...you have it setup as a secondary address on your router. Ok, to my knowledge you cannot set a ip helper address on a secondary address. I thought maybe you might have had the router interface broken into sub-interfaces.

So I can know a little more info, what model router and what switch do you have?

 

[OCCSteve]

I think it's a Cisco 3600 series. Without going down & looking at it, I'm not sure of the exact model. Currently, fe1/0.1 has 3 ip addresses on it, with no vlans. The 3rd address that I haven't mentioned is irrelevant. It's just routing a bunch of other addresses to a Layer 3 switch that we have. I can't get into the router myself because it is actually our ISP's router.

 

[cajuntank]

Dude... you just said the magic word, "Layer 3 switch".
To heck with trying to maintain sub-interfaces, secondary address, etc... on the 3600, just create you a couple of vlans on your L3 switch (one for each subnet), enable ip routing on the switch, and put the ip helper-address on the VLAN interfaces that are not on the same subnet as your DHCP server.

I would then make the switch your default gateway and just have a default route from your switch to your 3600. The 3600 will still have to have static routes for the other subnets.

 

[OCCSteve]

Okay, more background info: Our building is a convention center. We provide public IPs on multiple vlans to clients. The Layer 3 switch subnets them & has 3rd party equipment behind it for bandwidth management & other management of client connections.

Unfortunately, these other subnets are purposely outside the Layer 3 switch in order to provide an easy, open, DHCP backup in the event that internal equipment goes down(we have lost the Layer 3 switch 1 time).

We may be trying to do something that can't be done. It just seems like if both scopes are built in the Windows 2003 server, & both of their gateways are on the same flat network, then it should hand out IP addresses on either scope as it sees fit.

Thanks for the replies. I'll keep watching in case you or anybody comes up with a magic solution.

 

[cajuntank]

Thought about it some more and with the additional NIC in the server, your server is a router which adds complexity.

If you wanted to continue down that road, I would look at

http://www.tech-faq.com/dhcp-relay-agent.shtml

It seems that since this is a segmented network due to the two nics, you would have to enable DHCP relay to relay the DHCP to the other subnet (basically what you'd be doing on a L3 switch or router with the ip helper statement).

Otherwise, another thought would be to find some other box to be DHCP server handing out the addresses for that other subnet. As long as the gateway(primary and secondary address) can talk to both subnets, it should work. You just might have one pc get a 33.x address and a machine right next to it get a 36.x address.

I had this happen on a Novel network where the guy needed more addresses for a new building and just added a secondary address to the router. After he added a DHCP server for just that building and the network was not really segmented, it was just the luck of the draw on what IP address the machines in both buildings were assigned. I can say this, and I believe this was just some strange issue in this scenario, that when the machines got the "right ip address" for their building, everything was ok. When a machine got an IP address from the server in the other building, performance was slower. But again, I believe this was due more to the layout of the switches and router in relation to where the machines were. I eventually reworked this into a segemented network with VLAN(s).

Hope that helps.

 

[OCCSteve]

In the server properties, under bindings, only the primary address showed up. I found this article:

http://technet2.microsoft.com/windo...717f-4d63-a0fd-97e412667b7b1033.mspx?mfr=true

After reading it I just decided to go with the 2 NIC method. After statically assigning 36.2 to the 2nd NIC the DHCP automatically added it to the Bindings list. It seems to be working fine, and we don't really care who gets IPs from which scope. As long as they get one & can access the Internet, that's what they're paying for.

Thanks again for the many & prompt replies.

 

[cajuntank]

Glad you got it working.