DHCP excluded addresses still getting leased

[Patrick McHenry]

I just excluded a bunch of addresses from my 6509 AP DHCP scope and then rebooted my APs. They are still getting the excluded addresses from the scope. Anyone know why?

[Brian Wcisel]

I had a similiar issue once in the past.  In my case, bouncing the interface resolved the problem.

[Patrick McHenry]

bouncing the switchport?

[Brian Wcisel]

In my case it was the fastethernet interface on a 2911 router.  

[Patrick McHenry]

was that the interface that connected to the AP?

[Patrick McHenry]

I failed to mention that most of them did get the correct addresses.

[Brian Wcisel]

No there was a small switching infrastructure to which several AP's were connected, but the problem was prevelant across wired and wireless connections.

[Peter Paluch]

Hello,

I believe that this happens because in the lease database on your 6509, the IP addresses are still recorded as bound to the MAC or Client IDs of your APs. When a station requests an IP address, the DHCP server first looks into the lease database to see whether the station has already been assigned an IP address, and if not, it only then allocates a new lease, taking the exclude list into account. As the leased addresses were allocated before the exclude list was configured, simply configuring the exclude list and rebooting the clients may not help.

You will need to clear the lease database using the command clear ip dhcp binding X.X.X.X to remove the offending addresses.

Best regards,

Peter

[Patrick McHenry]

That did it but, why are only some of the APs keeping the excluded ones?

[Peter Paluch]

Patrick,

Did the keep their old IP addresses even after you cleared the DHCP binding database and reloaded them?

Best regards,

Peter

[Patrick McHenry]

no, they got new ones after I cleared them. Thanks for the tip. It just seems strange that only a

few kept their old address.

[Peter Paluch · Accepted solution]

Patrick,

I agree with that. In any case, a particular IP address leased to a client depends on various factors, including its MAC address, its Client ID (an opaque value), whether it already has a lease recorded in the lease database, whether the client itself remembers its last IP address and requests it directly on its next boot via DHCPREQUEST (as opposed to going from scratch via DHCPDISCOVER), etc. There are so many variables that giving a straightforward answer is probably not possible - it would require sniffing the DHCP traffic and debugging the DHCP server process on the IOS to exactly pinpoint what was going on.

In any case, thank you very much for your generous ratings!

Best regards,

Peter

[Patrick McHenry]

Thanks, Peter.