I have a network for a small company that has been running without an issue for a few years. We have Cisco IP phones on VLAN 100 and computers on VLAN 1. The VLANS are auto assigned by the Cisco managed switch (It detects the phones and sets them to the corresponding VLAN).

We have recently added a new work area to the company where a 24 port Netgear JGS524PE switch with VLAN support has been placed. I have the managed switch connected directly to the 24 port switch (to the same port as when everything used to work without an issue). When we first set it up, there were issues with computers intermittently not being able to obtain IP addresses from the DHCP server etc. I found out that the guy that made up the cables had not crimped many of them properly and some wires were not touching the contacts etc.

After fixing that, the computers work MOST of the time now. However, I have been testing it with another computer directly connected to the 24 port switch. I found that I was only able to obtain an IP address some of the time. So I set my computer to have a static IP address. When I try to ping the upstream managed switch, sometimes I am able to ping it and other times I am not able to. I can ALWAYS ping other devices connected to the 24 port switch however. When my computer is unable to ping the managed switch, the other computers are still able to communicated across it somehow.

I am not very experienced with setting up networks (as my day job is Software Development) and so I have never encountered this type of issue.

What could be causing computers intermittently not being able to get across to the upstream switch?

Possible causes that I can think of (but unsure of the likelihood) are:

  1. The VLANs are somehow causing an issue. (But would this stop me from pinging the managed switch?)
  2. There is still a bad network cable on one of the ports that is somehow confusing the 24 port switch. (Not sure if this actually can happen or not).
  3. The 24 port switch is faulty. (Even though it is brand new)
  4. Another device on the network is causing an issue that prevents other computers from communicating properly. (Not sure if this actually can happen or not).


  1. I have to set something extra up on the 24 port switch. (I only just found out that it actually can be managed via a web page).
  • Still bad cable, get a professional to test it with the proper equipment.
    – user36472
    Commented Mar 11, 2018 at 10:46
  • I am using a store bought cable for my computer to the switch. I also have a cable tester and have tested all of the cables to the other computers. Are you saying a bad cable is affecting the switch?
    – Asagohan
    Commented Mar 11, 2018 at 10:52
  • Modern network cabling is not a simple thing. You need solid-core horizontal cabling (up to 90 meters), and can use stranded patch cables, (up to 5 meters on each end of the horizontal cabling). There are many tests, and a proper tester costs $10,000+. See this answer for the basic tests. Installation is where most cable problems arise, and you must test the installed cabling.
    – Ron Maupin
    Commented Mar 11, 2018 at 17:11

2 Answers 2


This sound extremely like a botched cabling job...

I found out that the guy that made up the cables had not crimped many of them properly and some wires were not touching the contacts etc. After fixing that...

You need to have the links measured with a proper network tester. These testers check whether attenuation, crosstalk etc. are within the allowed margins. Any link that's not been properly checked is a gamble.

A simple continuity tester just tells you whether the cables are in the right places, no shorts etc. It can't tell you if the link really works.

On the switches, check the log for up/down events and check the interface counters for any errors, especially FCS, Alignment, and Tx drops. If any of these are higher than maybe a dozen (mainly from connecting/disconnecting a link) the link is unreliable.

Additionally, if you haven't done this already: you need to set up the VLANs on the Netgear as well. The uplink trunk has to be set up in the very same way as on the Cisco. The phone ports also need to be configured as VLAN trunks.

I'm not sure if you can set up the Netgear to automatically assign phones to the VoIP VLAN. Alternatively, most vendors have DHCP options that tell phones connecting to your PC VLAN to move to the VoIP VLAN instead. You just put those on your DHCP server.

  • OK. Thanks for the advice. I will try your suggestions tomorrow and see if I can get anywhere.
    – Asagohan
    Commented Mar 11, 2018 at 11:46

I wouldn't depend on automatic configuration for VLAN's (access port). It depends the vendor. And might be a security issue: A client should not be able to choose his VLAN, they need to be enforced at network level. And clients should not be able to access the switch management, on it's own management-vlan. Automatic VLAN configuration is only for trunks (switch to switch, etc).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.