I administer an unusual network that has weird DHCP behavior. The DHCP clients are almost all "appliances" that use a custom operating system and IPv4 stack, with a few Windows Embedded-based DHCP clients thrown in. Some of the design considerations and elements were:
* No more than 100 clients per subnet / VLAN
* 900+ appliances, making for at least nine subnets
* Cisco core switches acting as DHCP relay agents
* Redundant DHCP servers, alternating between 80 / 20 and 20 / 80 split scopes for each subnet
Getting the network running was quite the feat, but we accomplished it, being the largest Ethernet and IPv4 network I've had a hand in assembling.
Appliances get moved from subnet to subnet quite frequently, at least once a week. The eight day DHCP lease default is fine for this, but sometimes an appliance won't take an IPv4 address from the new subnet it connects to. Instead, it tries to renew (DHCPrequest) the address it had from its previous subnet, and the DHCP servers will gladly acknowledge the renewal if the logs and time stamps are to be believed. This makes the client unreachable, and I have to delete the client's old DHCP lease before it will accept a new address from a DHCP server.
I'm seeing this behavior with Windows PCs added to the network, suggesting this isn't specific to the custom OS and IP stack. It happens even on the simplest of networks, with a single Ethernet line from one of the cores to a client, so that rules out switching or wiring problems. Sniffing packets suggests the core switches are doing their job of relaying DHCP packets.
After beating this problem to death, I'm now wondering if it's possible to prevent a client from starting with a renew (DHCPrequest) and have it instead start from the beginning (DHCPdiscover). This might mean a client could get a different IP address every four days, but that's OK as we have dynamic DNS working and can talk to these clients by name. I can't change the behavior of the clients because it's a custom OS, but could I configure the servers to somehow ignore or reject renewals?
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