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RElay Agent

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  • RElay Agent

    I have a dhcp server with a addresss. I setup scopes with the , and addresses. I have a watchguard firewall that utitlizes natting. We have 2 subnet NATTEd from the firewall and All on the same wire. Here is the problem, workstation can recieve address but cannot receive 10.100.5 or 10.100.14 addresses. I am almost certain I need to setup a relay agent. Am I correct? Thanks for any help!!!!

  • #2
    Re: RElay Agent

    Please clarify.

    What IP setting does the client use? Isn't it simply in a different subnet?


    • #3
      Re: RElay Agent

      sounds like your router may not be effecvtively blocking broadcasts.

      is your dhcp server configured to listen and provide addresses on addresses in all subnet ?
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      • #4
        Re: RElay Agent

        DHCP Server IP addess = I configured a scope with the and a client can receive an address. If I configure a scope with a range, client does not receive an IP address. Client is on the same network as the server.

        Network consist of router connected to a firewall connected to a switch.


        • #5
          Re: RElay Agent

          if it's all just on the one switch, and not separated by vlans or routers, then I'd expect it to get that address.

          DHCP works on broadcast addresses.. it will broadcast a "discover" packet to
          the first server available that hears the broadcast will respond with an "offer" packet based on the subnet and interface the broadcast is heard on.
          the client then responds with a "request" packet
          and the server responds with an "accept" packet.

          you will need a relay agent on each of the nat'd subnet gateways.
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          • #6
            Re: RElay Agent

            Or use the IP helper command from the switch, if it has one that is.

            What type of switches do you use??


            • #7
              Re: RElay Agent

              thanks for the re;ies!

              I have cisco 3750 switches.... Because the DHCP server which has a ip address and is on the same wire as the clients, the client should be able to receive an offer from address? I was thinking it would not matter what ip address is sent from the server because the clients and server are on the same wire. However, the clients are not receiving the address;however, they can receive addresses. Now if they are on different physical subnet, it would matter unless an ip helper address is created. Am I right?


              • #8
                Re: RElay Agent

                if they are physically on the same segment, or the same broadcast subnet, call it whatever you want, but if they are physically on the same "wire" then they'll receive an ip address from which ever dhcp server happens to reply the fastest.

                Let me try this a differnt way.

                within my network here, I have one authorised DHCP server (it's a DC, but that's not the point)
                our network consists of 2 switches, 1 router, and 1 subnet.
                so i've got one cable from the router to the switch, and 1 cable between the switches, and everything plugs into the switch. they are simple, managed, layer 2 switches.

                my workstation starts up, tries to contact DHCP server, and reaches the domain controller, which says "ok, your broadcast is coming on subnet 192.186.x.0 and i'm authorised, so, here you go"

                now - my colleague is testing a consumer modem, for some reason, and plugs it into the same flat network. It has DHCP enabled, but on the subnet
                I boot my workstation, and it sends out a broadcast.
                The modem on receives it, and says "oh, here you go, is your address" because it was faster than the DHCP server and it's all on the same segment

                if I took the second switch from the example above, and plugged it into a different port on the router, and left it on the subnet
                and took the first switch, connected to the router and made it the subnet, then ensured there is no cable between the switches, the following would happen.
                we now exclude the modem from the scenario.

                My DHCP srever is on the subnet (siwtch1) if I plug my workstation into switch 1, it will get a dhcp address of
                If i plug my workstation into switch 2, all of a sudden, I get no DHCP address.

                if I add an IP helper on the router, forwarding traffic from network to the dhcp server on, my workstation gets a address, because that's the only scope available

                however, if i setup a second scope on the dhcp srever, say and I configure my IPhelper, it should then say "ok, i'm rebroadcasting from" and the dhcp srever should say "ok great, here's"

                i'm not 100% sure on how to configure an IP helper however..
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                • #9
                  Re: RElay Agent

                  I resolved the problem. After revisiting DHCP docs, I discovered that addresses had to be super scoped to work. I created a super scope and now all is working. Thanks for all the help!