I have AIX (5.3 TL5 SP4) workstations that use DHCP. The DHCP server is a Windows server running MS DHCP. Nothing fancy.
When Windows clients shut down, their IP address remains reserved as long as their lease time has not expired (you can still nslookup that host name and get an IP address back). On the AIX clients, however, as soon as dhcpcd is stopped, the IP address is immediately unregistered, so the host name becomes unknown to the network. I have so far been unable to figure out how to change this behavior.
Any suggestions?

This is in fact the way it is supposed to be. When the client shuts down it is supposed to release its ip address with dhcp release command to the server. Windows how ever ignors this behaviour (as well as many other things (",) ) and holds the IP address for the whole lease time.

Many thanks for the info. It's kind of funny that Windows ignores the expected behavior. I think it would make more sense for the Windows machines to follow the rules here, simply because most laptop computers are Windows based, and more likely to move from network to network regularly and NEED to get a new, unique address ecah time they move. Our AIX stations, however, are Intellistation model 275 and 285 (NOT very mobile). Because of "quirks" with the Windows DHCP server, rebooting these stations often leads to IP address conflicts, which require manual intervention on the part of the Windows admins to correct.
It hurts me deeply to say this, but ... ouch ... I'm trying to get it out here ... I wish I could make my AIX stations behave like Windows workstations, in just this one small way. That hurt. I'll go say my "Hail Mary"s now, and will try not to blaspheme again.

When have you known Windows to do the normal (Standard) thing?

What RFC states that the correct behavior at system shutdown or reboot is to release a DHCP bound IP address?

This is the good thing about adhering to the standards - there are always provisions to accomplish what you want to accomplish, whereas ignoring the standard often leads to unresolvable conflicts.

You said your AIX systems are immobile anyways, so you probably want to get them always the same IP address anyways. You can do so at the DHCP servers configuration by reserving a fixed IP for the AIX system. You could move this IP at the top or bottom of the DHCP-served address range and exclude it from that range accordingly. Don't use the DDNS means you probably use for name resolution of DHCP-based systems but reflect the quasi-static IP address with a static DNS entry. This way you get name resolution even if the system is shut down.

I hope this helps.

bakunin

Configure the Windows DHCP server to always allocate the same IP to the AIX machines' MAC addresses. Then there will be no collisions. I know this is doable as I've done it myself.