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DHCP Problems

Posted on 2011-09-21
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Last Modified: 2012-05-12
I have some major problems with DHCP on my network. To summarize, I have two subnets with two unique gateways for each subnet.

Scope 1
192.168.1.30-255 open
192.168.1.240

Scope 2
192.168.15.30-255 open
192.168.15.240 gateway

Here is my problem, packets randomly drop on my network. When I stop DHCP those same packets stop dropping. I was able to capture some packets using wireshark and found that the DHCP server is offering:

Paket 1 DHCP Offer 192.168.1.25 (DHCP Server address)
Packet 2 DHCP Discover 0.0.0.0
Packet 3 DHCP NAK 192.168.1.25 (DHCP Server address)

For some reason it appears the server is saying no

While that is occuring, packets drop network traffic. So I turn off DHCP, then all packet loss stops.

Can someone please help me troubleshoot? Is there any further information needed?

I moved this DHCP server last week to another server and imported all the settings from the old server. Things worked fine for 6 days.
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Question by:AxiomEmp123
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14 Comments
 
LVL 6

Expert Comment

by:joeyfaz
ID: 36577628
Ok let's start with simply and basic question: Are the 2 scopes on 2 separate physical networks?
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Author Comment

by:AxiomEmp123
ID: 36578217
No, the scopes are on the same physical network.
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Expert Comment

by:Fred Marshall
ID: 36578514
Yes, but you didn't tell us the subnet masks...  important!
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Author Comment

by:AxiomEmp123
ID: 36581509
Subnet Mask 255.255.255.0
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Author Comment

by:AxiomEmp123
ID: 36581514
Subnet mask on the NAK's showing up in wireshark show 255.255.255.255.
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Expert Comment

by:Fred Marshall
ID: 36582154
OK, so now I have:
192.168.1.0 / 255.255.255.0
with DHCP Scope 1or Range 1 of:
192.168.1.30-255 open
and Gateway:
192.168.1.240

192.168.15.0 /255.255.255.0
with DHCP Scope 2 or Range 2 of:
192.168.15.30-255 open
and Gateway
192.168.15.240

Well, the first problem I see is that the Gateway address is within the DHCP scope in both cases.  
That's not good practice.  So you might want to address the Gateway somewhere between .1 and .29

Also, .255 (the broadcast address) is included in the DHCP scope.
One would hope that the machine is smart enough to ignore that but let's assume that it causes the problem you're seeing.
Change the DHCP scope to .30 to .254 to avoid the network broadcast address altogether .. for sure.

What is the DHCP lease time?  I suggest 4 hours / 14400 seconds.
If it's too short then there might be a lot of DHCP traffic.  Odd but possible I suppose.  e.g. what if it's 1 second?
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Author Comment

by:AxiomEmp123
ID: 36582803
I followed your first suggestion for each subnet and excluded the .240 gateway address from distribution in each range.

My end address is .254 in each range not .255 as I originally suggested.

The current lease time is 8 days. I do not like that because we have around 90 apple devices that come on the network at random. BootP clients are set to 30 days.

I'm still monitoring as we speak and will follow up. Any idea on best practice for lease times?
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Expert Comment

by:joeyfaz
ID: 36583685
You cannot have 2 DHCP scopes on the same physical network. Period
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Expert Comment

by:Fred Marshall
ID: 36585244
joeyfaz makes a good point.  How do you separate the clients between subnets if there are 2 DHCP servers, one on each subnet space?
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Expert Comment

by:joeyfaz
ID: 36586605
Im not saying it can't be done, but the way DHCP works by protocol, it can't, not for the same network segment. What you CAN do is setup a DHCP failover, that will work. Can you run by us why you are looking to have multiple scopes?
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Author Comment

by:AxiomEmp123
ID: 36587588
We have too many devices for one subnet. range @192.168.1.X

That scope filled up with IPADs, Iphones, laptops all knocking one another off the network or unable to join at all. An ISA with RAS is running on the network with no defined scope so when those leases expire, RAS takes 10 it thinks are open whether they are being used  or not!

The guy before me setup a guest range @ 192.168.2.X.

So a subnet was created at 192.168.15.X in order to clear up conflicts. I began moving over clients with DHCP reservations from the .1.X network which has improved things some.

I'm also not sure if I'm explaining things correctly. There is one superscope with two scopes defined underneath within DHCP.

I've been considering the idea of creating a second DHCP server for the .15 subnet because it only accepts reservations I give it. It does not hand out addresses now.
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Expert Comment

by:joeyfaz
ID: 36588531
Convert the network to a larger scope. For example network address: 192.168.1.0 with a subnet mask of 255.255.254.0 will give you up to 512 hosts on the same network.  
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Expert Comment

by:Fred Marshall
ID: 36589564
Or, since you already are using
192.168.1.0 / 24
and
192.168.2.0 /24
and
192.168.15.0 /24
and assuming that you may have some static addresses in probably two of those subnets, change to:

192.168.0.0 / 20 or 255.255.240.0

This will give you 4092 host addresses from 192.168.0.1 to 192.168.15.254

that encompass all those you have now.  Then you will only need one DHCP server.

While 4092 may seem like a lot, it's not all that worse than 512 or 1024.  Surely you don't do this for security reasons!!
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Accepted Solution

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Fred Marshall earned 500 total points
ID: 36589578
If the guest subnet is physically different (as might be a wireless network) then take it outside the other subnet.

The attached paper addresses splitting up physical subnets in a simple way to limit access from one to another while providing internet access for all.
Multiple-Subnets.pdf
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