Sonicwall DHCP server – static reservations

This is just a quick post to highlight a quirk I found with the Sonicwall DHCP server (built-in DHCP server in Sonicwall firewalls).

I had an issue at a customer this past weekend where we had Cisco Access Points sitting behind a Cisco wireless controller and Cisco switch that pull their IP address from a Sonicwall firewall’s DHCP server.  We have reservations set up for each AP in the Sonicwall DHCP scope (each AP actually exists as a single entry scope as that is how Sonicwall handles DHC reservations; this is quite different from how DHCP reservations are handled in Windows) and the AP’s would not pull their address after a prolonged network shutdown.  I tried all sorts of things thinking that the problem had something to do with the Cisco controller but I did reboot the Sonicwall a couple of times, with no luck.

After some messing about I dropped in a small dynamic DHCP scope on the subnet that the AP’s are on and, BOOM! each AP pulled an address!  Weird.  I then dropped the dynamic scope and cycled the Sonicwall DHCP service which made no difference, AP’s still did not pull their assigned addresses.  On a whim I disabled all of the AP reservations then enabled just one, the assigned AP suddenly woke up and pulled its proper IP address.  I enabled the rest of the reservations and all of the other AP’s came back online with the correct addresses.  Double weird.

So, lesson learned.  Cycling the Sonicwall and even cycling the DHCP service didn’t “wake up” the static assignments but the individual disable/enable did.  If you hit a similar problem with Sonicwall DHCP reservations then this trick might help.