Platform: Windows Server 2008 (and R2)

DHCP and Classless Routes: In my environment I use DHCP option 249 under my Window Server 2003 DHCP servers to provide routes to my XP clients. I use a heavily subnetted environment where we don't have all routes on even classful boundaries (subnet masks of 255.255.0.0 or 255.255.255.0 don't exist here, we have stuff like 255.255.254.0).

This past weekend I ported my DHCP scope over from a retiring 2003 server to a new 2008 server, and everything seemed to work fine using the following KB: http://support.microsoft.com/kb/962355. However, my scope option 249 was missing, my clients were missing their routes, and I had an option showing up as unknown on the new 2008 server.

I did a bunch of research, and frankly didn't find much until I ran across a Technet discussion about scope 121 on 2008, which is identical to 249 in 2008 and before. The catch is that buried in the documentation is this little backwards compatibility hack that says if a client asks for 249 and the DHCP server doesn't have one, it should lie and respond with the details of scope 121 so the client gets the routes. I deleted the unknown scope and details, recreated the routes under scope 121, and tested with an XP client using a release and renew and it worked. *whew*

The flipside to this adventure is that I have a couple of Vista and Windows 7 clients we're playing with and they are not getting routes in some subnets. Those subnets are served by 2003 DHCP servers, which don't have the option 121. 249 is there, but Vista and 7 don't ask or care about 249, so they are blind to the additional routes. I'll be working to upgrade those subnets to a 2008 based server so everything is happy.

I can't explain the change, but make sure you consider that option in your 2003->2008 transitions so you can avoid the headaches I did as clients renewed their DHCP leases and lost routes.