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Wireless networking problem

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  • Wireless networking problem


    I have a wireless networking question for you. I'm pretty new to wireless networking and have had an issue which I don't understand.

    I provide IT support to a small charity that has a simple domain setup. Their office is hosted in a medieval building and has two floors. The walls are about three feet thick and made from stone.

    They have one domain controller which also acts as a file server. It runs DNS and DHCP. The building is cabled with CAT5. The client computers at the charity's location have static IP addresses and are connected via CAT5 cables to the network.

    However, they have lots of volunteer staff who use their own laptops, iPads, smartphones etc. when working at the office.

    In order to ensure that the extra volunteer staff working on both floors have full access to the Internet and to the local network I placed a wireless router on each floor. A single wireless router is not powerful enough to reach both floors. The wireless router on the ground floor (which is hard wired to the network) is the gateway. The router on the first floor simply accepts wireless connections and is connected via a CAT5 cable to a switch which in turn is connected to an RJ45 wall socket. Both wireless routers have the same SSID.

    They called me a couple of days ago and said the wireless network had stopped working. Devices were being assigned APIPA addresses. I visited their office yesterday and tried connecting my laptop to the wireless network but had the same problem. When connected via a CAT5 cable, everything worked properly. No problems were reported on the domain controller hosting DHCP - nothing was being denied.

    The gateway/wireless router downstairs was obviously functioning. When I checked the upstairs router I found that the switch to which it was connected was not powered on (the power cable had been removed). When I replaced the power cable the wireless network functioned properly and I was able to connect and obtain an IP address without any problems, access the Internet and the local network.

    I don't understand why the fact that the upstairs router was disconnected from the network should have affected wireless connectivity. The wireless router/gateway downstairs was working fine so I don't get why the wireless traffic was unable to contact the DHCP server and get an IP address.

    Here's a diagram showing how the network is configured:

    In reality, some of the clients on the ground floor are connected to the desktop switch and some are plugged directly into RJ45 wall sockets. The first floor plan is entirely accurate.

    I've done a little research this morning and have discovered that when using two or more wireless routers with the same SSID you should make sure that the channels used by each do not overlap. However, I'm still none the wiser as to why the wireless network failed, especially as the wireless router on the ground floor (and which I was sat next to when trying to connect), appeared to be working fine. This is the first time I've had a wireless network problem so I have very little experience of what the issue may be.

    Can anyone help me with this, please?

    A recent poll suggests that 6 out of 7 dwarfs are not happy

  • #2
    Re: Wireless networking problem

    From what you're saying, you were able to connect to the wireless network on the ground floor, but the client was unable to obtain a DHCP lease when the upstairs router was disconnected. This leads me to suspect that the upstairs router may be running a DHCP service.

    When a Windows DHCP server detects another DHCP server on the network, it simply shuts down. In your setup, such a scenario would only affect computers on the wireless network, as you're using static IP addresses on the other PCs.

    Try running ipconfig /all while your laptop is connected to the downstairs network and check the IP address of the DHCP server. I wouldn't be at all surprised should the IP address reported turn out to be that of the upstairs router.


    • #3
      Re: Wireless networking problem

      Thanks a lot for replying.

      No, when both wireless routers were connected to the network the wireless network was available.

      When the wireless router located on the first floor was disconnected from the physical network (the power to the switch had been disconnected), the wireless network was unable to direct traffic to the DHCP server hosted by the domain controller. This occurred when trying to connect wirelessly either on the ground floor or the first floor.

      Neither router hosts DHCP - I disabled it on both routers when I set them up many months ago after creating the AD domain.

      When connecting my laptop via a CAT5 cable to the network it was able to obtain an address from the DHCP server.

      The problem seems to be that because the upstairs router was unable to connect to the DHCP server (because it could not connect to the network) it someone 'broke' the wireless network.

      As I said, restoring power to the switch that the upstairs router was connected to also restored the wireless network.

      I would have expected any devices connecting upstairs to be unable to connect because there would have been no way for the traffic to reach the network.

      I also expected that any devices connecting downstairs would have worked fine.

      I'm sure there's a good reason why this happened, but I'm certain it is not because two DHCP servers were live.

      If you have any further thoughts please post.

      Thanks again.
      A recent poll suggests that 6 out of 7 dwarfs are not happy


      • #4
        Re: Wireless networking problem

        OK, I missed the part where you were able to get an IP address by plugging your laptop into the switch.

        An IPv4 DHCP lease is obtained using layer 2 broadcasts. There's no way a broken link between the ground floor and first floor could affect broadcast frames on the (still working) LAN on the ground floor.

        So, these are the facts:
        1. Clients on the wireless network on the ground floor couldn't get an IP address when the physical link between the ground floor and the first floor, meaning that the link between floors is somehow involved in this process.
        2. By plugging a computer directly into the LAN on the ground floor, a DHCP lease could be obtained. This means the DHCP server is clearly connected to the LAN on the ground floor.
        3. The wireless router on the ground floor is directly connected to the LAN segment on the ground floor.
        4. Reconnecting the switch on the first floor fixed the problem.
        Assuming these facts are correct, I conclude as follows:
        • When you were unable to obtain a DHCP lease, you couldn't have been connected to the LAN on the ground floor
        • This means means your laptop couldn't have been connected to the wireless AP/router on the ground floor (fact #3), but instead must have been connected to another wireless AP/router without a layer 2 connection to the LAN segment on the ground floor
        • The only (known) wireless router/AP affected by the status of the switch on the first floor, is the wireless router on that floor
        Conclusion: Your PC must have been associating with the router upstairs. Either that, or the router on the ground floor is somehow connected to the LAN in such a way that it is affected by status of the switch upstairs, which according to your diagram seems extremely unlikely.

        You can easily troubleshoot this by disconnecting the LAN cable on the router on the first floor. Wireless clients on the ground floor should not be affected by this, unless they actually associate with the wrong router.


        • #5
          Re: Wireless networking problem


          This is what I was wondering. I was considering changing the name of the upstairs router to see if this was the case. However, before I started providing support for these people they had a single wireless router downstairs and the signal strength was not enough to provide a reliable connection upstairs (it was really, really weak - but I suppose a connection can be made).

          Also, when the switch was powered off three separate devices made the attempt to connect to the wireless network whilst located in the same downstairs room that the wireless gateway is located in. I had also tried disconnecting and reconnecting whilst in that room with the same result.

          I was wondering if it was sheer fluke that all the devices were connecting to the wireless router upstairs and then wondered if changing the name of one of the routers would be the best solution.

          Given the facts as you state them I agree that the only answer is as you say and that the devices were in fact connecting to the upstairs router. Although it was powered on, it was not connected to the physical network and thus could not contact the DC to get a DHCP assigned address.

          The next time I visit their office I will rename the upstairs router so that this does not happen again. If they are unable to connect via one router they can disconnect and try the other.

          Thanks again for your help.
          A recent poll suggests that 6 out of 7 dwarfs are not happy