dhcp

[kevin_lane]

we have 2 servers which do dhcp which control our 10.25.8.0 scope

our lease address range from 10.25.8.0 - 10.25.11.254 (council network attached)

 

im not sure how this works but on 1 of our servers we are running out of dhcp address quite fast. am i right in saying if a client askes the dhcp and then dhcp says no shouldnt the dhcp pass this on to the next dhcp server also how would i know if we have a dhcp relay server. i wasnt really involved with the setup i just feel somthing bads going to happen and i feel like this is the start of somthing

[tmcd35]

AFAIK, the client broadcast a request for DHCP. One of the servers will respond and send out a valid IP. We have 3 DHCP servers and it seems quite random which one is going to respond, and I don't know how the responding server stops the others from responding. But if a server as run out of IPs, it won't respond to the request leaving the other to take up the load.

 

We are in a position now were two of the three DHCP servers are constantly full and I've never seen any problems with IP allocation.

 

I'd only worry if BOTH DHCP servers where running out of address. Then you'll have clients that can't connect to the network.

[Jon]

This article should explain all you need to know.

 

http://www.microsoft.com/technet/prodtechnol/windows2000serv/reskit/cnet/cncb_dhc_ogjw.mspx?mfr=true

[kevin_lane]

strange... we do have lease time that should release ip address each morning so we have more ggrrr i dunno. its just weird becasue i've noticed this problem the other day and i went down to sort a problem with a laptop which keeps d/c today from the network and it has like an ip address of 10.25.8.247

 

what happens if i delete all of the address lease in dhcp

[tmcd35]

If you delete all the address in the lease, you run the risk of IP conflicts as the computers with IP address will not request a renewal of their lease until the lease is up. The DHCP will not know that the IP is in use and reallocate it. conflict!

[kevin_lane]

well i guess i dont want to be doing that....but thats weird you should say that becasue i seen our backup server a have an ip conflict the other day. i told my boss and he said dont worrie about it. i was bit :s i mean i know what they are but still that shouldnt be happening on a backup server. im going for my next exam 70-291 and im still finding it hard to understand about subnets nvm so what should i be looking out over the next week or so if this problem still goes on more network connectiviy problems ? on our other server we have 176 ip address left. i think im going to have to do some reading tonight thanks

[Jon]

Did you read the article on the 80/20 rule?

[tmcd35]

Are the two DHCP servers dishing out address from the same range? If they are then this could be causing you problem. Back up server with IP conflict, and laptop dropping connection.

 

On our setup, our range is 10.96.232.0 - 10.96.235.255.

 

DHCP1 allocates 10.96.233.0 - 10.96.233.255

DHCP2 allocates 10.96.234.0 - 10.96.234.255

DHCP3 allocates 10.96.235.0 - 10.96.235.254

 

The 10.96.232.1 - 10.96.232.255 range is reserved for static IP's.

 

If both of your servers are allocating in the entire range 10.25.8.1 - 10.25.11.254 then they can, and will over allocate IPs and cause conflicts. You need to make sure the range is split between the two servers...

 

DHCP1 10.25.9.1 - 10.25.10.255

DHCP2 10.23.11.0 - 10.25.11.254

 

for instance.

[Jon]

At risk here of sounding boring but...

 

Did you read the article on the 80/20 rule?

 

http://www.microsoft.com/technet/prodtechnol/windows2000serv/reskit/cnet/cncb_dhc_ogjw.mspx?mfr=true

[tmcd35]

Don't know about the OP, but I read it. It says nothing about how DHCP actually works. It just details MS's recommendation that 80% of the addressable scope is leased from on server and 20% of the scope is addressed from the other.

 

Personally, I prefer the split on my network (33/33/33). That way if one DHCP server fails, 66% of the address are still available rather than 20%

 

The ultimate outcome it the same - DHCP redundancy.

[Jon]

This article may explian it a bit better...

 

WindowsDevCenter.com -- Ensuring DHCP Server Availability

[tom_newton]

Far better to have a second dhcp server "idle" where the dhcp service is only brought up as needed. May need to copy the leases file occasionally, but even if you don't it's better than the shocking kludge that is 2 servers.

 

Edit: Ooh, looks like the wondrous ISC dhcp server already has support for failover:

http://www.madboa.com/geek/dhcp-failover/

 

If you can't do something similar with windows i'd be vaguely surprised. I'll ask Robf tomorrow if he hasn't died of Wales-Lurgy.

Edited September 22, 2008 by tom_newton

[PiqueABoo]

Far better to have a second dhcp server "idle"

 

Yes, you can use "netsh" to fully configure DHCP, export leases to a file and import from a file.

 

Not that I'd bother unless I inexplicably had a lot of reservations - Windows DHCP is easy to install again somewhere else and can be set to do conflict detection.

Edited September 22, 2008 by PiqueABoo
Hit post much too early.

[tom_newton]

Pique: Ah, didn't realise windows DHCP did conflict detection - if that's the case, you're right - there's no point copying leases, which makes life 100 times simpler! I would have thought it's be pretty easy to set up a script to repeatedly ping (or ask for the same DHCP lease?) the primary, and start/stop the service on secondary as appropriate. I'm a *nix man myself tho - so i'm not gonna try that on win!

 

If you were feeling really clever you could sniff unacknowledged DHCP reqs, but that sounds like too much fun.

[tmcd35]

Far better to have a second dhcp server "idle" where the dhcp service is only brought up as needed. May need to copy the leases file occasionally, but even if you don't it's better than the shocking kludge that is 2 servers.

 

Edit: Ooh, looks like the wondrous ISC dhcp server already has support for failover:

Failover with ISC DHCP

 

Having had three DHCP servers running here with no problems for the past 2 years, I have to ask what is wrong with multiple DHCP servers on 1 LAN if they are correctly configured to avoid IP conflicts?

 

I read both Big-Jon's links which gave general advise but no answers and unfortunately your link is blocked here!

 

Surely by splitting our IP's across 3 servers (33/33/33) we achieve greater redundancy and reliability should one of the servers fail?

[DMcCoy]

Pique: Ah, didn't realise windows DHCP did conflict detection - if that's the case, you're right - there's no point copying leases, which makes life 100 times simpler! I would have thought it's be pretty easy to set up a script to repeatedly ping (or ask for the same DHCP lease?) the primary, and start/stop the service on secondary as appropriate. I'm a *nix man myself tho - so i'm not gonna try that on win!

 

If you were feeling really clever you could sniff unacknowledged DHCP reqs, but that sounds like too much fun.

 

It doesn't, by default. Although you can turn it on iirc. Splitting the range 50/50 also works well, I used to do that, although with 40 odd subnets it's easier to have just the one and restore the db to the second DC for now

[tom_newton]

tmc: nothing "wrong" but it is a bit of a hack, it isn't deterministic, by which I mean you can't guarantee what's going to happen.

 

Also, if you have a fairly dense IP population, you will find that if a server fails, you may run out of space. However, with 3 like you have, you'll at least have 66% left.

[PiqueABoo]

It doesn't, by default

 

Windows DHCP doesn't hand out addresses by default.

[tom_newton]

Windows DHCP doesn't hand out addresses by default.

 

Probably a good thing - got to be careful in networking products to have the user make positive choices in order to run services etc.

[DMcCoy]

Windows DHCP doesn't hand out addresses by default.

 

Doesn't check for duplicate IP's by default. There's an option for it