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Solved: Switches and DHCP

Discussion in 'Networking' started by Doghnut, Jan 18, 2011.

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  1. Doghnut

    Doghnut Thread Starter

    Jan 18, 2011
    Apologies if this a repeat -- I searched the forum and found no duplicates.

    I run a smallish LAN at work (30-70 computers depending on the day). I am having a problem with connecting to my DHCP server through an unmanaged switch.

    I have only one ethernet port in a particular room and need to connect 3-4 computers through that port. It is my understanding that an unmanaged switch should do the trick (correct me if I'm wrong).

    The problem I have is that I cannot get more than 2 computers to get an IP from DHCP. It's a different two computers everytime -- it just seems that it's a race to the DHCP server and once two computers get there, the others are locked out.

    Various things I have tried:
    -Static IPs have no bearing.
    -Forcing a NIC to use reduced speeds makes no difference.
    -The switch is Auto-MDIX
    -The problem persists whether using regular switch ports or the uplink port.
    -I have also tried a second switch I own with the same problem.
    -Windows firewall is disabled and turning off anti-virus makes no difference.
    -My scope has not run out of IPs.

    Is there something I am missing here? I feel like the problem is simple or fundamental and I'm just not seeing it.

    Thanks in advance for any help you can give.
  2. PedroHin


    Aug 13, 2010
    That is very strange. Right now, I am as puzzled as you are.

    With this setup, how many switches are there between the affected PCs and the router/gateway?

    From the room with the 4 PCs, how far is the next upstream device (switch/hub/router)?

    I guess you already know that as a last ditch, you can go with the DHCP reservations, and then statically configure each of the PCs for the DHCP server's reservation? With this setup, I would be interested in knowing if the 4 PCs can then all simutaneously ping the gateway IP (ping -t [gatweway'sIP]).
  3. Doghnut

    Doghnut Thread Starter

    Jan 18, 2011
    Sorry for the long time in response -- I appreciate the help Pedro.

    Of course, I had this problem for weeks and as luck would have it, I figured it out as soon as I posted here. It turns out that each of our ports runs through a set of Cisco switches which are much more intelligent than I previously thought. I had to configure each port to accept the correct connection.

    I will mark this thread as solved in case anyone else ever experiences a similar problem.
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