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  • Dhcp

    I'm currently running WINSERV2003 PDC, I'm getting ready to install a sceond PDC as a backup / failover. The question I have is the first server has a DHCP scope 1-200. Can I setup the second PDC with the same range or will they both send out the same I/P's to different workstations and couse a conflict. They are both on the same domain.

    Thanks

  • #2
    Re: Dhcp

    Running 2 DHCP servers with the same scope is not possible since Windows Server 2003. This because there can be only one server authorotive for one scope. This is the idea behind DHCP scope authorities.
    If one DHSP server detects an other server running the same scope, it will check who is athorotive in AD. The one who isn't, will shut itself down to prevent dupliate IP's the handed out. You can however run two DHCP servers besides each other, but each authorotive for its own scope (No Overlapig).

    Two DHCP servers are quit commen, but the scope is devided between the twe servers.
    40% of the scope on server A, and 60% on DHCP server B.
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    • #3
      Re: Dhcp

      ok, One quick question, mos to the Scope is being used, like 3 -135 and 145-201 and 202 - 250 area reserved. Do I have to release those i/p's needed to create the 80 / 20 rule? and if the server with 80 percent of the I/PS fails will the 20% server be able to handle all the request if 80% of the scope is excluded?

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      • #4
        Re: Dhcp

        It all depends on you DHCP settings.
        The lease time is a very important factor here.
        If you set the lease time to one hour, you know your clients will start to release there IP's at maximum one hour. If one server goes down, and the remaining server can't hande out more than 20% of the IP's needed. You will know that only 20% will recieve a IP address when the lease time has expired.

        Now, by default the IP lease time is set to 7 days, which gives you a couple of days to repair the failing server should it go down.

        Maybe its handy to understand what the DHCP client does.
        When the client starts it well aquire an IP address from a DHCP servers (basically the one who responds the first to client requests will hand out the IP). When the lease has reached 50% of its time, the client will try to renew that lease. If the DHCP server is not available, it will keep trying (time intervals) untill the lease time avengually expires. When the lease time expires, and still no DHCP server respnds, it will release the IP address and use a APIPA address (169.XXX.XXX.XXX).
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        • #5
          Re: Dhcp

          Also you could just install dhcp on the failover and not activate it. If the other one fails then just activate the backup.
          Please remember to award reputation points if you have received good advice.
          I do tend to think 'outside the box' so others may not always share the same views.

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          • #6
            Re: Dhcp

            If you are on a single subnet a 50/50 rule can be implemented if you have monitoring solutions in place to alert you if any of the Services or Servers are down (Although since you have it installed on a DC, DHCP should be the last of your worries).
            If you apply the 80/20 rule then depending on the volume and structure of your organisation if the DHCP that is configured to lease 80% of the scope is down, then you are going to strugle to avoid downtime if the one with 20% has leased all his available IPs.
            In reality the clients can contact any DHCP available on the subnet so the second server has probably leased a few IPs already. That's why the 80/20 rule works better by providing redudancy in two different subnets (presuming the router has been configured to forward BOOTP packets).
            IMHO loadbalancing DHCP servers is a bit of an overkill and creates to much admin overhead.
            I have found Backup and restore works better for me.
            The DHCP database %SystemRoot%\System32\Dhcp on DHCP servers contains information about DHCP leases and reservations. By default, this database is automatically backed to %SystemRoot%\System32\Dhcp\Backup every 60 minutes. What you need to back up, however, is the configuration of your DHCP server, so that if the server bites the dust you can restore this configuration to a replacement DHCP server. To back up the configuration of a DHCP server, use the netsh command:

            Code:
            netsh dhcp ServerIP dump > dhcpconfig.dmp
            This creates a netsh script called dhcpconfig.dmp, which you can copy to your replacement server and run to configure this server by:

            Code:
            netsh exec dhcpconfig.dmp
            Cheers
            Last edited by L4ndy; 24th October 2008, 15:29.
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            • #7
              Re: Dhcp

              Originally posted by Killerbe View Post
              It all depends on you DHCP settings.
              The lease time is a very important factor here.
              If you set the lease time to one hour, you know your clients will start to release there IP's at maximum one hour. If one server goes down, and the remaining server can't hande out more than 20% of the IP's needed. You will know that only 20% will recieve a IP address when the lease time has expired.

              Now, by default the IP lease time is set to 7 days, which gives you a couple of days to repair the failing server should it go down.
              One thing to bear in mind with reducing the lease time to one hour though, is that it'll generate loads of Broadcast traffic and also the default lease time is 8 days.

              Ta
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              • #8
                Re: Dhcp

                Originally posted by L4ndy View Post
                One thing to bear in mind with reducing the lease time to one hour though, is that it'll generate loads of Broadcast traffic and also the default lease time is 8 days.

                Ta
                Define "Loads of broadcast traffic". We often reduce the lease time to ten minutes on smaller sites, at least temporarily, when they run out of addresses; and a lease time of an hour doesn't seem to impact our bigger site networks at all. As long as you have one DHCP server, and helper addresses correctly configured, it's pretty good...


                Tom
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                • #9
                  Re: Dhcp

                  About 4 packets per client
                  DHCP Discover packet
                  DHCP Offer packet
                  DHCP Request packet
                  DHCP ACK packet

                  If the client can find the DHCP server in a timely matter
                  Marcel
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                  • #10
                    Re: Dhcp

                    Originally posted by Stonelaughter View Post
                    Define "Loads of broadcast traffic". We often reduce the lease time to ten minutes on smaller sites, at least temporarily, when they run out of addresses; and a lease time of an hour doesn't seem to impact our bigger site networks at all. As long as you have one DHCP server, and helper addresses correctly configured, it's pretty good...
                    Adding to what Dumber described below there is also the Unicast traffic during the renewal process at 50% of lease time and if for some reason the client fails to renew it's lease for the first time there is another DHCPDISCOVER broadcast packet sent when around 90% of lease time is reached.
                    Not suggesting that will bring the network to its knees, but undoubtly will have an impact on the network however "Invisible" it may seem.
                    Plus add to that the workload if DDNS is enabled...

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