I thought I was going a bit mad when this first happened to me and it is an issue that has cropped up several times for colleagues as well. After configuring an 1142 Autonomous AP for a static IP address the AP then reverted to DHCP. This seems to have happen within a minute of configuring the static address on the BVI interface and moving on to other configurations.
The resolution is simply to re-apply that static configuration and (I advise) write to memory straight away. After applying the configuration to the BVI interface twice this seemed to stick. When pasting in a bulk generic configuration I’ve also advised people to make the BVI configuration the last bit they do which seems to work first time so I’m not entirely sure what is triggering it to revert! One theory is that if a Lightweight AP does not detect a heartbeat from its Controller it may revert to DHCP in order to try and locate a new Controller to join and this attribute has affected the Autonomous mode as well.
What to do if you’ve already sent the AP out to a site
Panic ensues! Fear not, this can be remedied and – double whammy – this fix can also work if you have forgotten to write your configurations to the startup-config before sending the AP off to a remote location.
If you have your AP sitting on a VLAN with no DHCP server then performing a show cd neighbours detail on the switch it is attached to should reveal the AP minus a management IP address. For an AP with no configuration it will likely just show up as ‘ap’ again with no IP.
If you have a VLAN with a DHCP scope linked (e.g. a general user VLAN linked to a server) then simply put the AP onto that VLAN and waits for it to obtain an address. Once it has, telnet into the device and configure it back to a static address et voila!
For those hoping for a more local resolution you can always configure a temporary VLAN with a DHCP scope based on the switch from which the AP can obtain an address from. Once you have logged in and made your configuration changes just remove the VLAN and scope from the switch. I’d advise ensuring your switch is in VTP Transparent mode when doing this to keep the VLAN local. If the switch is in client mode you will be unable to make the temporary VLAN.
Note: As soon as you configure that static address you will lose comms with the AP so you’ll need to move it back to its proper VLAN and reconnect on the new IP.
I will try to put up some screenshots of the configurations when I have time to. After a quick search on the Cisco Forums didn’t reveal much it seemed like a post was in order! Something worth noting is I have been using quite an old IOS image (124-21A) and this may be the culprit.