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I am a computer science teacher in a small school who is getting to grips with a role as server/network admin. Today, we all lost internet connection and were unable to access the server remotely.

It turned out a main switch an (HP2530 series 48port) in the server room had failed and luckily we had another similar one sitting around. I switched the connections over and we are up and running except:

Many users are having difficulty connecting using DHCP, I suspect the DHCP server has stopped giving out IP addresses. Our server is running Windows Server 2012.

Are there any suggestions of how to troubleshoot this issue? I can now access the server, both remotely and physically. I connect devices to the network if I use a static IP but I would prefer it to just work via DHCP.

Thank you all...

A DHCP service restart hasn't fixed the problem so I will look into resetting the replacement switch in case there were existing settings affecting the DHCP service.

I have also now reset the "new" switch back to factory settings but the problem still persists. Users can join network but do not get an IP address unless they manually set one. Strangely this affects Mac users more than Windows users. Windows users who have existing IP address continue to be able to connect while Mac users have to add IP address manually.

Wireshark shows that DHCP does advertise itself but Apple and Linux devices are not getting IP addresses over wireless. I get a warning from the DHCP event log "This computer has at least one dynamically assigned IPv6 address.For reliable DHCPv6 server operation, you should use only static IPv6 addresses." Could this be related to the problem?

I have also tested a Windows 10 device, it is able to release and renew its IP address using DHCP. All the Apple devices in the school which are mostly BYOD and so have various configurations are not able to get an ip address via DHCP over WiFi or wired connection. Linux devices are having the same problem although previously connected devices were initially able to connect over Ethernet. This is not normal, does anyone know what is the difference between Unix-based devices and Windows in this regard?

This is proving a real problem, we called in the IT comapany who originally set up our network to log in remotely, set up our new switch and check our system. They can't work out what the problem is either. We can't work out why Apple devices can't connect via DHCP and Windows devices can.

Simon Baldwin
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    Need to check for the DHCP server role on the server. Look in server manager or administrative tools. If it is there you can restart it from there. If restarting it doesn't help, it is possible the switch could of been doing dhcp relay or the new switch is missing some other configuration – Drifter104 Jan 08 '20 at 12:43
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    Do you have a copy of the configuration file from the failed switch? Did you start with a factory reset configuration on the replacement? Chances are there were things like VLANs setup that could affect how the DHCP server is connected to the network. – Bert Jan 08 '20 at 12:55
  • Do you think it would do any harm if I just reset it? It is connected to our Sonicwall and the wifi network manager device. – Simon Baldwin Jan 08 '20 at 13:34

2 Answers2

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  • Do factory reset of that switch, see will that solve your problem
  • get a small 5 port, "dumb" switch and connect it directly to you DHCP server and test it, if problem persist, Download Wireshark and check does your DHCP server advertise itself.
  • Check event logs on your DHCP server

Come back here :)

Leshy
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  • Factory reset - problem persists but at least no new ones :) – Simon Baldwin Jan 09 '20 at 12:18
  • Wireshark shows that DHCP does advertise itself but Apple devices are not getting IP addresses. I get a warning from the DHCP event log "This computer has at least one dynamically assigned IPv6 address.For reliable DHCPv6 server operation, you should use only static IPv6 addresses." Would this be enough to mess it up? – Simon Baldwin Jan 09 '20 at 12:26
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    @SimonBaldwin Only Apple devices, Windows devices obtain IP address normaly? If so that's not normal?Your Apple devices are switched to dynamic IP are they not? Maybe the network settings are corrupt on your Apple devices, try reseting them.Have you ckecked DHCP logs on this location c:\windows\system32\dhcp" and in Event Viewer. How complicated is your DHCP, does it have a lot of rules, scopes if not remove that role and install it again. There are some steps that you can do without reinstall, but logs should help with that. You could try to repair DHCP database with JETPACK, or CLEAR it.. – Leshy Jan 10 '20 at 07:38
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    @SimonBaldwin _"This computer has at least one dynamically assigned IPv6 address.For reliable DHCPv6 server operation, you should use only static IPv6 addresses. Would this be enough to mess it up?"_ - I doubt that. – Leshy Jan 10 '20 at 07:40
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    @SimonBaldwin _"what is the difference between Unix-based devices and Windows in this regard?"_ - None, DHCP is based on RFCs. – Leshy Jan 10 '20 at 13:13
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Apologies if this isn't the case, but normally when I see this it's because the clients have come up in AutoIP mode after the switch failed.

The solution is to force the clients to renew their IP connections. Either order them to renew, or physically connect/disconnect them.

user165568
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  • Thanks for answering this. The company I worked for has now folded (unrelated to this issue!). We never did solve this as it folded in June 2020 and all the network infrastructure was sold off separately. This DCHP issue remains a mystery... – Simon Baldwin Feb 24 '23 at 09:15