Yes there are LOA's setup for this kind of thing. All the others Centers/Tracons around us was working just fine. Even Memphis ATCT was normal. An example of the LOA in the area I work in Which is the "new west" RZC approach only owns up too 15,000 feet and we own above it. Well when we went ATC-0 RZC approach took over the airspace up to FL230. ZFW took FL230 and above for the area. So this is the same throught out the center. ZKC, ZFW, ZID, ZHU, ZTL, MEM, BNA, JAN, RZC, LIT...etc were all involved with airspace take over in yesterday's mess. The crazy thing was we still had radar just no freqs or land lines. So pretty much everything was taken care of by the use of a big no no....are cell phones.