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DHCP across subnets?

C

Citadel535

Senior member
We are planning on subnetting our network and are trying to figure out a DHCP server could be used. We have three subnets, one for employees, one for students, and two for anything in the future.

If we have a router (anything even a PC) with two IP addresses assigned to one NIC that is routing traffic through the employee and students subnets. Now, two PCs, each to be on different subnet are connected to a switch that connects to the router. The DHCP server is connected to the router on the third subnet.

Both PCs are on the same broadcast domain so when they send a DHCP request, how does the router know to send it to the DHCP server so that both PCs are assigned to the proper network? We can't do MAC reservations as we are really talking about 2000 PCs.

Is this even possible? If not, how is this usually handled?

Thanks in advance for the help!
 
wow 2000 PCs talking to each other through a single multi-homed router? Step 1, add another nic. now.

step 2, use dhcp relay - it does exactly what you need:

win2k - dhcp relay agent guide, very good and informative read - http://www.microsoft.com/technet/treeview/default.asp?url=/technet/prodtechnol/windowsserver2003/proddocs/standard/mpr_how_dhcprelay.asp

cisco - dhcp relay / ip helper, pretty close to what you're doing - http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a00800800ed.html

linux - google for linux dhcp relay and you get about a 40,000 hits ... seems like "dhcrelay" is the answer here.

freebsd - it's a port, very easy to install & use: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/network-dhcp.html

cheers,
randal
 
Hey thanks. Yes our network has 500 employee PCs, 500 student/lab computers, and 150 printers, all running IP and IPX and on one huge subnet! Let's jam some more traffic on there!

Actually we are getting a new router with multiple NICs in it.

But if the student PC(meant to get a student subnet host address) and the Employee PC (meant to get an employee subnet host address)will be connected to the same switch and then connected to the same multihomed NIC, how will the router/DHCP server know to give the PCs the right address for the right subnet?

Is this even possible?
 
I've never done it on a *nix machine before, but if you have multiple NICs doing the broadcast->unicast DHCP requests, the DHCP server will see requests (hopefully) coming from the different MACs of the router, and be able to discriminate from there.

The easy solution is to put multiple interfaces on the DHCP server, put it on multiple networks, and then decide what IP to lease by what interface the request comes in on -- I have that running right now in multiple places, works great.

randal
 
citadel,

You probably have some serious networking issues. 500 hosts on a single broadcast domain (ip subnet) will crawl.

Try to keep it under 200 stations if you can.

Any dhcp server will work, but on the router you have to tell it where to forward DHCP requests to.
 
Citadel535, get some real managed L3 switches (Cisco, Extreme, Foundry, etc.) and use VLANs. This will provide traffic separation and IP routing (some can even do IPX). You can then pull a trunk .1Q tagged into a PC and have DHCP assign IP addresses from different subnets to clients on different VLANs.

If you don't want to spend money, you can use separate dumb switches to do VLANs the hard way. But if you've got 2000 hosts, you need to seriously work on the manageability or you'll go crazy.
 
VLANs aren't really an option as we've read that they are being depricated by Cisco. We have Cat 1600s at the access layer, Cat 2600 XLs at the distribution, and Cat 5500s at the core. They are connected to a PIX 500 which connects to our 2600 router that goes to our ISP.
 
2900 and 5500 switches support VLANs. It would be realitively easy to segment your network for better performance.

A few 1600s in a single VLAN, trunked back to the 2600 switches, routed in the 5500.

You may want to seek professional help on this.

-edit- the entire network world revolves around VLANs, they certainly aren't going anywhere.
 
A 3550 will actually give you better routing performance than your 5500 ever could. If you're trying to segment (and I still recommend keeping it under 200 PCs, 100 or less in a campus environment) you could use a 3550 to do all your routing.
 
Plenty of Networkers out there looking to help you VLAN that network...(cough)

RE:" VLANs aren't really an option as we've read that they are being depricated by Cisco."

Man, and they tricked me into learning all that...dang..<G>
 
Citadel535, perhaps you saw the statement that Cisco is obsoleting their early ISL VLAN technology in favor of IEEE 802.1Q VLAN technology and that's where you got the idea that VLANs are obsolete.

spidey07, Cisco has a 3750 or something newer out that I think is now shipping, it's basically the next gen of the 3550, and should be used in preference. The usual Cisco chain yank. 5500s do VLANs, but if memory serves they have some serious implementation flaws (i.e., under certain conditions, packets can bleed from one VLAN to the next).
 
yeah, I was thinking more along the lines of routing performance of the 5500 and backplane limitations. only like 3.2 Gig I think.
 
yeah, I was thinking more along the lines of routing performance of the 5500 and backplane limitations. only like 3.2 Gig I think.
 
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