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Several DHCP Servers

Posted on 2008-06-13
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Last Modified: 2013-11-09
I have a simple network, all on one subnet (192.168.64.?). The main router has two wireless routers connected to it.

Normally I'd leave the dhcp servers of the two wireless routers off, and the main router assigns addresses 100 to 150. Does it hurt anything to say, instead have all three routers with their DHCP servers turned on, and have them spread the range out? E.g. main router assigns from 50-100, wireless router 1 from 101-150, wireless router 2 from 151-200? If you turn DHCP on on the wireless router, does it block any DHCP packets from relaying through to the main router?

The reason I ask is because I want the wireless routers to gracefully not allow any more than about 10 clients to connect, and I can't do that when the IP's are handed out by the main router. I'd like the wireless routers to manage their own IP addresses for the wireless clients.
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Question by:Frosty555
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10 Comments
 
LVL 5

Expert Comment

by:jenkinsme
ID: 21781154
In order for this to work you would have to create different subnets for each wireless router otherwise to DHCP servers would conflict with each other and none of your users would obtain an IP address automatically. You would want to set a static IP address of 192.168.64.xxx on each of your wireless routers and that would be their WAN address. Then allow them to supply a seperate subnet to their wireless clients that way the wireless clients could access through the WAN port your main router and subnet.
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LVL 3

Assisted Solution

by:Karl12347
Karl12347 earned 100 total points
ID: 21781172
It should be fine for each DHCP server to host a portion of the scope. Just make sure you dont misconfigure each DHCP scope incorrectly before making them all live. ie dont overlap the scope on two servers. Could cause major problems.

Hope this helps
Karl

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Expert Comment

by:Karl12347
ID: 21781194
I totally dissagree with jenkinsme, as long as you configure the ip addresses you want to lease on each device it will work a treat.

karl
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Expert Comment

by:jenkinsme
ID: 21781204
Karl are you sure about that? If you have a server that is giving out ip addresses and you connect a router and enable DHCP it will continually shutdown the DHCP service.
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Expert Comment

by:DocCan11
ID: 21781578
yes he is correct.. if you have multiple servers they can all run DHCP. The key is to not have the scopes overlap of you will have duplicate IP problems..
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LVL 44

Accepted Solution

by:
Darr247 earned 200 total points
ID: 21783688
I agree that you CAN run mutliple DHCP servers on the same network as long as their scopes have no overlap (though the practice is generally not recommended), but I'm not certain what you want to do is going to work. I think once the 10 you want to allow have connected and the 11th connects the 'full' router is just going to relay the DHCP request on up the line and the request will be filled since it's from the same subnet.  But I won't mind being wrong on this... if it works the way you want, please let us know. :-)
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LVL 6

Assisted Solution

by:DocCan11
DocCan11 earned 200 total points
ID: 21785309
If you understand how DHCP works it will help you understand the problem.. DHCP uses UDP BROADCAST packets to get an IP address. Since the packets are broadcast ALL DHCP servers that get the broadcast will offer an IP address to the client from their scope. If there are not addresses left in their scope they will not respond. The client takes the FIRST DCHP offer it receives and broadcasts an acceptance back.. This is heard by all DHCP servers allowing each server to act accordingly (if you did not accept an offer from a DHCP it will remove the temp reservation it made). There is no REAL easy way to ensure a client will get an offer from a particular DHCP, even reservations will not ensure this. The only way is to use switches to isolate the DHCP traffic in the proper way and then use DHCP relay agents to allow backup.

The other concern seemed to be raised here is DHCP servers with addresses from the same subnet, this is not a problem as has been pointed out because DHCP servers do not talk to each other.. That is why you must be so careful when you setup the scopes.

I hope this clears this up
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LVL 31

Author Comment

by:Frosty555
ID: 21787426
okay, I do appear to be running multiple dhcp servers now on the network. My laptop is getting an IP from the wireless router's DHCP server. What I'm not sure of, is if enabling the dhcp server on the wireless router will cause it to filter out and not allow dhcp broadcast packets from going through. If it does, that would be perfect, but there's no way to know until I max out the users on the wireless network. I will give that a try soon.
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Expert Comment

by:Darr247
ID: 21788596
If it turns out they are getting through, if the wireless unit has a built in firewall you might be able to filter ports 67 and 68 and still allow other broadcasts. Whether the firewall would block those ports from its own DHCP server is another question.
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LVL 31

Author Comment

by:Frosty555
ID: 21831445
Nope, they do get through. As soon as the one dhcp server stops working, another one kicks in. The port blocking doesn't seem to be doing anything but the router's interface for that is rather confusing so I might not be doing it right.

So it looks like having several DHCP servers is not a good way to limit the number of users on a particular network device. Seems like having more than one dhcp server is actually pretty useless unless you need the redundancy.
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