Lately we've been experiencing some issues with DHCP on our network. Clients (Windows 10) will connect, either via Wireless or Ethernet, then might need to wait around for 5-10min before receiving a DHCP address from the server. However, at other times they'll receive the lease immediately. If you configure the address manually, things obviously work fine.
Wireless clients are just bridged to the LAN, and so are communicating with the same server as Ethernet clients, via the same routes. The DHCP server is a virtual machine on another subnet at another physical location, the path to which traverses a few Cisco FTD Firewalls. All of them have been configured with Fastpathing for DHCP traffic, but it has not made a difference to DHCP performance. QoS is configured on both the server's LAN and the clients' various LAN's. STP is not the culprit either, as the link is up, and wireless clients are also affected.
I performed some packet captures on the DHCP server and a couple of test clients, and found that the clients are indeed receiving their DHCP offers. No suspicious looking error logs, and once the client does pick up the DHCP address, it is the one contained in those DHCP offers. I found that upon running the "netsh winsock reset" command on the clients then rebooting, they pick up the address right after booting. Upon another reboot, or running "ipconfig /renew," it again takes 5-10min to get that same lease back. Disabling AV had no effect on things.
I have confirmed that routing is fine, ACL's are not blocking anything, and that pings from the troubled LANs to the server are not dropping and have excellent ping times of 1-5ms.
Any ideas what this could be? Narrowing it down is killing me. We have recently been making a lot of changes on the network, from switches to firewalls to AV products, etc, so it's hard to say any one change is what started it.
