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DHCP Server Win 2016 full arp cache

So one of my peers setup a 2016 on a VLAN/Subnet. He came to me the network guy to try and figure out why only 40 or leases had been assigned to his VMs. The subnet is a /23 with 512 potential addresses and 20 or so reserved. There's no helper address here - just a party on the VLAN. So I hopped onto the DHCP server to make sure he configured the
IP address OK etc. But what surprised me was when I ran arp -a I found like 450 addresses associated with MACs already. The DHCP manager showed only 40 or so assigned. Anyone have a thought how all of these addresses might have gotten assigned to the mac addresses while the DHCP server failed to make a note of it? I pinged a few of the addresses from the arp -a and could not reach them. These are VDI desktops to-be. If the addresses are not pingable DHCP should go ahead and assign an address right? It's not concerned with what's in the arp cache right? Or no??
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System Administrator
Distinguished Expert 2018
I've experienced something similar with the DHCP on the Cisco switch. For some reason, it suddenly stopped assigning IP addresses although only a few were used. Turned out that the arp cache on the switch was full. The solution was to clear the arp cache on the switch.

To clear the arp cache on Windows Server, run:
netsh interface ip delete arpcache

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Also, I would check the EventViewer for any related events.

Btw. Make sure that "Allways dynamically update DNS records" and "Discard A and PTR records..." options are checked in the DHCP console.