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  • Which DHCP Server is a phone using

    Hi,

    Anyone know a way to tell which DHCP server a phone is getting its IP address from?

    I did a INFO+# but it doesn't show this info.

    I tried plugging a laptop in to the Ethernet jack to see if the laptop might get an IP address and then IPCONFIG would show it - but it didn't work.

    Any thoughts?

    Thanks,

    Rob

  • #2
    You'll have to plug in to a hub or mirror the port on your switch, then do a packet capture using something like wireshark. Once you capture the packets during a boot, it is just a matter of seeing which server sent the DHCP offer.

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    • #3
      Plug your laptop back into the phone and do an IPCONFIG /ALL

      That will show you the DHCP server address.

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      • #4
        Right, that should work if you aren't using VLANs. But since he said in the OP that didn't work, I figured that probably wouldn't work.

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        • #5
          Originally posted by aclements View Post
          Right, that should work if you aren't using VLANs. But since he said in the OP that didn't work, I figured that probably wouldn't work.
          In the original post he said he did an IPCONFIG which doesn't display the DHCP server, IPCONFIG /ALL will.

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          • #6
            I know im just pointing out the obvious here but how about just looking at the leases in all your dhcp servers. I assume you have 2 that are load sharing. All the other ideas work as well though.

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            • #7
              the problem may also be one that I have run across...a site with 2 different DHCP servers offereing different scopes (Not by design...ie, rogue DHCP server). I had an issue where a phone kept getting a 192.168.1.x ip addressm but my laptop would always get a 10.132.5.x from DHCP. Really odd, that even on the same cable, the phone always got the wrong one.

              Naturally, the phone can't do a IPconfig -all to display it's lease server, and the laptop is getting DHCP from a different source.

              Somewhat easy fix though. Pull the network cable on the known DHCP server, and refresh your lease on the laptop. It will pull from the rogue dhcp server, and you can do an IPCONFIG -all and locate it. Probably a backup dsl modem offering it up.

              Charles

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              • #8
                Okay - sorry it took so long for me to get back to it.

                I did some digging - we found a DHCP helper IP address in our VoIP switches, and it is pointing to the DHCP server we are using.

                So the issue is that the DHCP server never gets the Lease info back from the switch.

                So somehow int he DORA (discover, offer, request, acknowledge) the Phones get an IP address - but the switch wont' tell or the DHCP won't listen to the IP address assignment.

                We get a phone that glitches and I am wondering if the DHCP is handing out the same address - its rare and we have 200+ IP addresses available so it may be why its so rare - that all the phones tend to renew their leases, etc so they don't get many IP conflicts.

                I will let you know what I find out from the switch vendor's tech support.

                Thanks,

                Rob

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                • #9
                  What are you using for DHCP service? If it is Microsoft, make sure conflict detection is set to 1 or higher.

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                  • #10
                    Dhcp

                    aclements was on the money.

                    conflict detection will prevent dup ip handouts. If you reboot the dhcp server without conflict detection, it will start handing out all the addresses that were already given out.

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                    • #11
                      Originally posted by rbergin View Post
                      So somehow int he DORA (discover, offer, request, acknowledge) the Phones get an IP address - but the switch wont' tell or the DHCP won't listen to the IP address assignment.
                      Is the DHCP server connected to a switch port that is a member of more than one VLAN?
                      If the DHCP server is in the data vlan, the switch port must be untagged in the data vlan and must not be a member of any other vlan.
                      This post has some DHCP troubleshooting: http://www.shoretelforums.com/forums...vlan-sort.html

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