DHCP Pool Selection

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USMCGrunt

2[H]4U
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Mar 19, 2010
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Going through some planning for moving from a decentralized network with servers at multiple sites to a centralized network. I currently can't remember....or may have never known...how to set it up so that PCs at a specific site get IPs from a specific DHCP pool. Right now, we have five sites, each on its own subnet, I would prefer to leave it that way. I would have DHCP installed on a single server and it would have five separate pools on it but how would the server know which pool to assign to each host coming in from a single connection. All sites would be inter-connected with point to point connections but that would all be transparent to the server right? I know routers have a DHCP helper command to push DHCP requests across links but that command doesn't have anything more than just an destination IP right?

Am I more or less stuck with having to redesign the network into a single subnet? It's a relatively small network...there's just a lot of different things I would have to touch, I'm sure you guys understand how much of a pain that could be.
 
Is there a need to do centralized DHCP? If the connection between sites goes down, then your addressing will die as well and not even internet access is operational...

How are the sites connected? Direct Point-Point connections or via a VPN over the internet?
What resources (if any), will still be at the remote sites?
What resources will the remote site users access from HQ?
 
Vlan 5 at site 1 has IP range
10.1.5.x /24

Vlan 3 at site 4 has ip range
10.4.3.x /24

Your DHCP server at central office is
10.0.2.50

interface vlan x (at site x)
ip helper-address 10.0.2.50
(assuming cisco switches)


Then on the DHCP server just make a scope for each of those subnets.
 
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Pretty much what cyr0n_k0r said. DHCP server will have its own IP addy that everything will point to relay wise. Each subnet scope is really its own network.

So 10.1.5.x /24 for Site 1 would relay to the IP address of the DHCP Server (10.0.2.50) at the router/switching level. Server would get it and you're probably asking what it would do from there to figure out what IP pool it would belong to? Well, that's why you configure the VLAN with an IP addy. It puts it in the network that it was intended.
 
When the remote VLAN sends the request to the DHCP relay server it has listed on its interface, it includes its subnet in the request. The server sees that and hands off the scope that matches that subnet, done and done, enough details are contained in the request packet for the server to understand it.
 
Ahh yes VLANs, forgot about that part of the equation thank you.

The thought is to have EVERYTHING centralized with internal traffic running over PTP 10Mbps connections and then internet traffic would be routed out on a per site basis through the local ISP. Currently, we have everything running across our ISP using VPN tunnels for internal traffic. Unfortunately, the ISP here is incredibly poor with, maybe, a 90% uptime across the network. When the ISP goes down 4 of the 5 sites lose their ability to use email or access server shared files. Additionally, their backup plan is basically non-existent, I would like to have a NAS at two sites, each of them storing a copy of the network backups for redundancy but transfer speeds between sites currently sits around 128Kbps....vastly below speeds required to backup 300GB of data they currently have.
 
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