Dividing the Network
The first question you must always ask when contending with a routing protocol that provides multiple levels of routing (such as OSPF and IS-IS) is: Where do I divide up the network? The answer to this question predetermines many other design problems and solutions, so you must answer it carefully.
In IS-IS the network is divided up into areas, with level 1 (L1) routing taking place within the areas and level 2 (L2) routing taking place between the areas. L1 routers understand the topology of only the area they are within, whereas L2 routers know how to route packets traveling between these areas. (See Appendix B, "IS-IS Fundamentals," for more information on how IS-IS works.) The critical issue is where to put these boundaries ...
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