there are a lot of isis info you can find in the internet, but trying to be objective i describe you the most important.
If you have experience with ospf area segmentation, isis works almost the same way, instead you define level1 area for the routers to establish adjacencies, and level 2 only, or level1, and 2 for inter area comunication, so the border area router must belong to area 1 for comuniction with routers within his own area, and with level, 2 for comunication with another area border router who belongs to another area.
thus, preventing the flooding of large routing tables that belong to any area, to propagate and coverge only with his area, and not propagating to the entire network (as OSPF also).
hey men, I suggest you to use ospf or either closed BGP if you have veery big routing tables, and also for protect the performance of the area
