Strange Network Connectivity Issue

Posted on 2009-04-23
Last Modified: 2013-11-09
A user cannot login. I check the pc and it has no ip. I swap out the lan connection from the connected computer next to it and it still won't connect. Hmmm, looks like a bad nic. I take it back to the server room and plug it in. It connects immediately and works perfectly.
I am going to take a laptop that I know works, back to the room and plug it in. Then see what I get for a connection. I did notice that when I tried to connect a switch in the room it would not work properly either. Out or 4 computers I had connected, only 2 would get a connection. It is at times like this that I wish I would have spent some money on a good network analyzer tool.
I think that there is a problem with the wiring in that room. Of course, this is the president of the schools son, who is next inline for the top job. Murphy's law of network
Question by:gpfrank
    LVL 12

    Assisted Solution

    I'm confused, so you plugged 4 computers (where only 2 got a magic light) into where?  The Switch port in the server room?  Or into the jack in the wall at the desk?
    Option or Scenario 1)
    If it was the wall jack then it is possible that you have a bad or loose connection in the wall.  I assume you changed the LAN patch cable from wall to PC.  It could also be a tail (copper wire that is punched into jack but someone didn't cut off or install properly and is shorting out sometimes by rubbing another pair/conductor).  Open the jack and check it out..
    Option/Scenario 2)
    If it was 4 computers didn't work in the server room, then are you plugging them all into the same exact switchport?  is it a managed switch?  What brand?  
    You could try this on the switch if its a Cisco  
    Switch-A> (enable) set spantree portfast 2/1 enable

    Warning: Port fast start should only be enabled on ports connected to a
    single host. Connecting hubs, concentrators, switches, bridges, etc. to a fast
    start port can cause temporary spanning tree loops. Use with caution.

    Spantree port 2/1 fast start enabled.

    Link =
    LVL 7

    Expert Comment

    A quick wire quality check is to force the network speed to 10MBPS (at the switch or the NIC).  if that works, you probably have some kind of wiring issue.


    Author Comment

    IThe problem with the switch was in the classroom that the computer was in initially. I assumed that the wall jack was bad at first. The jack has 2 RJ45's. I connected the switch to the good side (the one that had a connected computer on it) then I was just going to connect both of the PCs to it and bingo, I'm good. That's when it got stupid. I connected the 2 computers to the switch and the 1st one STILL would not pull an ip. That's when I figured it was the nic. Took it to the server room to add a nic (it has an on board one) and plugged it in and it worked. WTF. No matter what the outcome. I am ordering a network analyzer. I don't usually do anything past the wall jack. I would call a cable company. It's just that this acts like a nic. I will test it tomorrow with my laptop.
    LVL 7

    Accepted Solution

    Could also be a defective patch cable on the 1st one?  Or bent pins in the RJ-45 port of the switch?

    You need a disciplined process to check every step along the way, patch cable, port, NIC.  

    It is also not impossible that he switch failed.  What happens when you connect the PC to the wall jack?  With the patch cord on PC #2?  You might want to make a cable of all of the tests. :-)
    LVL 3

    Assisted Solution

    Quick and easy way to tell. Set the spanning tree portfast command on all interfaces that have PC's directly connected. See if the problem goes away. If it does not, then install wireshark on the pc's. Boot them as usual. On the ones that do not connect, startup wireshark and take a peek at what is happening on those ports. This will give you a sure fire way of determining if you have a cabling, switch, or dhcp issue that needs resolved.

    Author Closing Comment

    Sorry for the delay. I am putting this on the back burner for now. I have a 46 computer rollout to do right now. I will take all information into consideration when I get back into that room. Thanks!

    Write Comment

    Please enter a first name

    Please enter a last name

    We will never share this with anyone.

    Featured Post

    Looking for New Ways to Advertise?

    Engage with tech pros in our community with native advertising, as a Vendor Expert, and more.

    Suggested Solutions

    This tutorial will go through the steps required to write a script that will back up the configuration settings of a HP-ProCurve switch. You will need to get the following things to follow this tutorial: Telnet Scripting Tool e.g. TST10.exe …
    Every server (virtual or physical) needs a console: and the console can be provided through hardware directly connected, software for remote connections, local connections, through a KVM, etc. This document explains the different types of consol…
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

    737 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    22 Experts available now in Live!

    Get 1:1 Help Now