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In our small company, we have VMs on ESXI. One of them being the Domain Controller.

We have Avaya phones with their IP being (192.168.0.11), our IP range is mutually exclusive from Avaya.

Our Desktops are picking that DNS and Default gateway up rather than what the DHCP gives. This only started occurring last week. Restarted the DHCP server and didn't fix it either. It's only affecting some PCs.

ipconfig /renew and /reset gives it the DNS and default gateway from DHCP. But is then reset to the Avaya DNS after restart. For the mean time, I have manually given the default gateway and DNS for those problematic computers (instead of automatic) and everything works fine, but still want to resolve it.

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  • Do you have a different VLAN for your Avaya phones?
    – eKKiM
    Apr 21, 2021 at 8:16
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    We don't have enough information on your setup to really begin with this. As previously stated, are the phones on a different VLAN? Do they utilize the same switchports as the PCs in question? Do you have separate ports for phones and for PCs, or do you do pass-thru on the phone to get Ethernet to the PC? What does your DHCP configuration look like, especially any Option strings?
    – Ackack
    Apr 21, 2021 at 19:06

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DHCP requests get contained in the same broadcast domain as the client, so somehow your clients requests are reaching the Avaya DHCP server and its replies are being picked up before the Windows server.

DHCP requests and replies are broadcast so will be recieved by any device whatever IP address and subnet mask you have set.

It is not sufficient to have a different IP range - you need to properly segment your network so the Avaya phones and Windows clients. If your switches support it, you can achieve this with VLANs. If not you can do it old school with different switches and cables for phones and PCs.

And if that is not possible, you probably need to share the same IP subnet with PCs and computers, and have a single DHCP server in place configured to assign different settings according to some property of the client making the request.

But since you mention this started recently, most likely you have somehow bridged two VLANs or broadcast domains. This could be a switch (physical or virtual) that has ports connected on both VLANs or broadcast domains.

As other people have said, more info is really needed, but hopefully this has helped you narrow it down a bit.

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