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Windows Thread, Options on spliting DHCP scope in Technical; We have a DC which runs DHCP and has been for a few years now. We use the range 192.168.12.0 ...
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    tosca925's Avatar
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    Options on spliting DHCP scope

    We have a DC which runs DHCP and has been for a few years now. We use the range 192.168.12.0 to 192.168.16.255 At the moment 192.168.12.1 to 4. are excluded from the range for servers. At the top end of the range i have over 60 reservations which start from 192.168.16.255 and come down.

    I want to split the scope across two servers, but because of the way i have it set up already with the reservations i m not too sure what to do for the best.

    Should i delete the reservations and start again with having 50-50 reservations on each scope.

    Suggestions please?

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    ChrisH's Avatar
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    Re: Options on spliting DHCP scope

    You can save yourself some work and dont delete the reservations until you do a backup through the DHCP console. You can then edit the resultant file in a text editor and delete half the reservations if you wish or make two versions of the file etc for import. Infact you can use that one file and save yourself a lot of work by just changing it to split the scopes etc an import everything back as you want it. It will save you fiddling with the GUI and the file format is easy to follow.

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    Re: Options on spliting DHCP scope

    All sounds good Chris but to be honest ia m not tha experienced at messing with DHCP really, one of those, if it aint broke dont fix it scenarios.

    I'll look into what you said.

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    Re: Options on spliting DHCP scope

    Just do a backup and have a look at the file in a text editor. The information for the reservations is probably one per line so easy to edit. The same goes for the IP ranges you will be able to guess how to edit the range. Dont worry its nice and straight forward.

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    Re: Options on spliting DHCP scope

    I want to split the scope across two servers
    I honestly don't think it's worth doing that, just makes it more complex for very little and very debatable gain (in most common scenarios at least).

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    Geoff's Avatar
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    Re: Options on spliting DHCP scope

    Using the 80/20 rule for splitting DHCP servers up is usually done for reliability and reliance purposes. If you have or want a redundant network infrastructure DHCP needs to be addressed just like any other single point of failure.

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    Re: Options on spliting DHCP scope

    Mmm.. that's what they all say.. ;b

    Is DHCP-20 on a different subnet from DHCP-80 and depending on (DHCP relay) latency to prevent leasing addresses until DHCP-80 dies? Are your leases long enough for DHCP-20 to have enough addresses during an outage (non pingable gateways could bite you here). If they're on the same subnet with a 50/50 split do you have at least twice the number of addresses you need and are you using superscopes?

    YMMV but a DHCP outage shouldn't affect too many machines. If those routers are pingable then Windows DHCP clients carry on with their existing lease until it expires, which in most sensibly configured cases should be quite a while. Laptops starting up for the first time after returning from foreign networks (e.g. home) are the main concern.

    Complexity isn't good for reliability either. I prefer one DHCP with a standyby i.e. a turned off DHCP on another box with the same configuration but with address conflict detection enabled.

    Yes that requires someone with half-a-clue to start the standby DHCP service, but the org is probably already screaming loudly for "someone" because of other stuff served from the dead DHCP box. How many people in education run DHCP-only boxes?

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    Re: Options on spliting DHCP scope

    Got a couple of PM's asking me to explain the 80/20 rule. So I figure I should.
    [*] DHCP server 1 has its scope configured to lease 80 percent of your available IP addresses (for example, 10.0.0.1 to 10.0.0.200).[*] DHCP server 2 has its scope configured to lease the remaining 20 percent of your addresses (10.0.0.201 to 10.0.0.250).

    Next, you make sure that the available addresses on server 1 are sufficient for all DHCP clients on the local subnet, and place server 2 on a neighbouring subnet. When a client tries to renew its lease by sending out a DHCP Renew message, this message typically reaches the DHCP server on the local subnet (server 1) first, but if server 1 is down then the DHCP server on the other subnet (server 2) can act as backup (provided your router is configured for BOOTP forwarding so it acts as a DHCP relay agent to pass DHCP broadcast traffic from one subnet to the next). For the other subnet you do the opposite; that is, server 2 provides 80 percent of the addresses for the subnet it resides on and server 1 provides 20 percent for the same subnet.

    The idea behind splitting the addresses 80/20 is because the 80 percent of addresses is normally sufficient for all the addresses needed on a subnet, and DHCP leases are typically three days, so if your subnet's main DHCP server goes down for a few hours then it's unlikely that more than 20 percent of the machines on that subnet will need to renew their addresses during the downtime, making the 20 percent pool of addresses sufficient.

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    Re: Options on spliting DHCP scope

    Complexity isn't good for reliability either. I prefer one DHCP with a standyby i.e. a turned off DHCP on another box with the same configuration but with address conflict detection enabled.
    I quite like the sound of this, is anyone else doing this method.

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    Re: Options on spliting DHCP scope

    and DHCP leases are typically three days
    That's a bit I have a problem with.

    Unless adresses are scarce (which they don't need to be given rfc1918 & NAT) or you have an unusually unstable network, then why shouldn't it be much longer? What's wrong with making them long enough to comfortably survive the summer holidays?

    Short leases are obviously good for some untrusted subnet where visitors/students can connect laptops wirelessly or similar i.e. you don't want machines you might never see again squatting on a lease for the next three months. Short leases might be good if you're forever shifting machines into different subnets, but who amongst us does that?

    it's unlikely that more than 20 percent of the machines on that subnet will need to renew their addresses during the downtime
    Yeah, but servers & services always try their very best to fail when no one is watching... a three day lease won't survive a bank holiday weekend (unless you're into wasting electricity occasionally starting up and shutting down machines in empty rooms).. and you've probably got more than 20% being started at the crack of dawn on the first day back by WOL... which won't keel over if there's an opportunity to humiliate you.

    is anyone else doing this method.
    You'll be lucky because "80/20" comes with the word "rule" glued to it (remember to do the same for any meme you want to dominate the planet).

    If you dig deep enough, there is or was an MS doc discussing the various pros & cons of the standby method and all the other approaches to DHCP discussed in this thread. Can't recall if that covers clustered DHCP, but that's another option.

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    Re: Options on spliting DHCP scope

    Here's another approach then. Configure both DHCP servers on your network with identical scopes. Now, normally this would cause problems because you might end up having the same address leased by both servers, causing clients to hiccup. However, if you enable address conflict detection on your DHCP servers and set it to 2, then each DHCP server will test an address three times to make sure it's not already being used before leasing it to a client, and this will prevent one server from leasing an address that the other server has already leased.

    The only downside to this approach is that it will make DHCP traffic on your network a bit more noisy, but DHCP traffic is so minimal anyway this will have only marginal impact on overall network performance. But why not set this to one attempt instead of two attempts? Safety, you don't want a network glitch that causes brief packet loss to ruin your setup.

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    Re: Options on spliting DHCP scope

    Quote Originally Posted by Geoff
    However, if you enable address conflict detection on your DHCP servers and set it to 2, then each DHCP server will test an address three times to make sure it's not already being used before leasing it to a client, and this will prevent one server from leasing an address that the other server has already leased.
    Have you actually tried this Geoff?

    Presumably this would only work if all your PCs are always switched on and respond to PING (which I guess is how DHCP server looks for conflicts). If PC1 gets IP address 172.16.1.10 from DHCP server A and is then shut down, PC2 could get the same address from DHCP server B. If PC1 now comes back on, it will be in conflict with PC2.

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    Re: Options on spliting DHCP scope

    Yes, you need to allow ICMP Ping on your client machines firewalls (if you are using them).

    If a client machine detects an IP conflict and is configured for DHCP then the first thing it will do is ask for a new lease.

    The 'blacklisting' of used IP's on DHCP server's is only lost on reboot.



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