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  • DHCP over Subnets

    Dear All,

    I have one dhcp server for subnet a and b, at the subnet b i have a cisco 1721 that relay the IP from my server at subnet a. The problem is when I move a laptop from a to b or from b to a, the PC receive the same IP like before (from the other subnet) and cannot connect to the network, how can I solve this problem?

    Regards,

    nito

  • #2
    Re: DHCP over Subnets

    Tried rebooting, or doing a ipconfig /renew ?
    ** Remember to give credit where credit is due and leave reputation points where appropriate **

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    • #3
      Re: DHCP over Subnets

      Do search for DHCP Relay agent.
      Leave 'reputation' when deserved :

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      • #4
        Re: DHCP over Subnets

        Manually perform an ipconfig/release and an ipconfig/renew on a computer on subnet B. If you don't get a valid ip address or get errors then you should put a packet sniffer on subnet B to see if DHCP packets are crossing the router. This will help you focus your troubleshooting. If you don't see DHCP packets through the router than you need to look at the router config again, specifically the ip-helper address config. If you do see DHCP packets then you can focus on the client and server.

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        • #5
          Re: DHCP over Subnets

          My Dears,

          How to solve the problem on each laptop when is moved is not really the problem, even ipconfig /renew dont solve it, I need to create a reservation with the mac then ipconfig /reneew. But I cant ask to the users do it all the time....

          For the dhcp-helper, sure is well configured, since is working fine normally with the reservations I make for subnet b, the clients receive the right IP, and I can see the all IPs released at subnet b in my dhcp server at subnet a. So the question is how to solve the problem in the all superscope.

          waiting for more ideas

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          • #6
            Re: DHCP over Subnets

            do use different vlans?
            Marcel
            Technical Consultant
            Netherlands
            http://www.phetios.com
            http://blog.nessus.nl

            MCITP(EA, SA), MCSA/E 2003:Security, CCNA, SNAF, DCUCI, CCSA/E/E+ (R60), VCP4/5, NCDA, NCIE - SAN, NCIE - BR, EMCPE
            "No matter how secure, there is always the human factor."

            "Enjoy life today, tomorrow may never come."
            "If you're going through hell, keep going. ~Winston Churchill"

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            • #7
              Re: DHCP over Subnets

              no vlans at this moment.

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              • #8
                Re: DHCP over Subnets

                Originally posted by nitoglycerine View Post
                no vlans at this moment.
                ahhhh. VLANs are your friend.

                you will need a relay agent to help if you dont have VLANs configured.

                the relay agent will look at the source IP of the DHCPdicover packet, and assign the address according to the scope defined for that interface.

                that will allow you to separate the scope amongst the network properly.

                RRAS isnt installed with 2003 by default, so install it and configure it. you can use this little walk-tru (very basic):
                http://www.computerperformance.co.uk...elay_Agent.htm
                Last edited by James Haynes; 20th February 2008, 15:31.
                its easier to beg forgiveness than ask permission.
                Give karma where karma is due...

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                • #9
                  Re: DHCP over Subnets

                  Originally posted by nitoglycerine View Post
                  no vlans at this moment.
                  Are the different subnets connected to same physical network?
                  If yes: create different subnets and use an IP-helper on the switch.

                  Otherwise it won't get to work because the DHCP Discover and other DHCP packets are broadcasts.
                  Marcel
                  Technical Consultant
                  Netherlands
                  http://www.phetios.com
                  http://blog.nessus.nl

                  MCITP(EA, SA), MCSA/E 2003:Security, CCNA, SNAF, DCUCI, CCSA/E/E+ (R60), VCP4/5, NCDA, NCIE - SAN, NCIE - BR, EMCPE
                  "No matter how secure, there is always the human factor."

                  "Enjoy life today, tomorrow may never come."
                  "If you're going through hell, keep going. ~Winston Churchill"

                  Comment


                  • #10
                    Re: DHCP over Subnets

                    Sorry, I misunderstood your original quote. It seems that DHCP works correctly on each subnet, the problem is when a computer moves from one subnet to the other it doesn't release the ip address from the first subnet, right? Well if it doesn't do it automatically you could use a login script or batch file in the startup folder that does an ipconfig/release and ipconfig/renew.

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                    • #11
                      Re: DHCP over Subnets

                      Sorry, I misunderstood your original quote. It seems that DHCP works correctly on each subnet, the problem is when a computer moves from one subnet to the other it doesn't release the ip address from the first subnet, right? Well if it doesn't do it automatically you could use a startup script or batch file in the startup folder that does an ipconfig/release and ipconfig/renew.

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                      • #12
                        Re: DHCP over Subnets

                        If you are using Windows DHCP then you should make sure that both your scopes are not in a superscope. This seems to have caught a lot of people out (including us). We mistakenly took a superscope as just a way of grouping scopes together, but it isn't, and we had exactly the same problem.

                        From Mike.
                        Last edited by Mikey405; 25th February 2008, 15:26.

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                        • #13
                          Re: DHCP over Subnets

                          make sense, I'm using the superscope, ill change my config and update the people here

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                          • #14
                            Re: DHCP over Subnets

                            YEES

                            You're right this is the solution.

                            Very Nice Mikey, one short post with one big solution, I hope you continue here in this forums to help me and the others.

                            nito

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                            • #15
                              Re: DHCP over Subnets

                              Good catch Mikey405
                              I never thought he would use superscopes.
                              Marcel
                              Technical Consultant
                              Netherlands
                              http://www.phetios.com
                              http://blog.nessus.nl

                              MCITP(EA, SA), MCSA/E 2003:Security, CCNA, SNAF, DCUCI, CCSA/E/E+ (R60), VCP4/5, NCDA, NCIE - SAN, NCIE - BR, EMCPE
                              "No matter how secure, there is always the human factor."

                              "Enjoy life today, tomorrow may never come."
                              "If you're going through hell, keep going. ~Winston Churchill"

                              Comment

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