Routing Hierarchies

The key to building large networks is to introduce a logical hierarchy. Problems related to complexity and scale can be addressed with the proper use of hierarchy. Having a hierarchical network can provide you with many benefits-most notably, route summarization and reduced SPF calculation-thus giving your network a faster convergence time.

OSPF was the first major routing protocol to support hierarchical networking (areas) within a single routing domain (AS). OSPF supports two levels of hierarchy: a backbone area (Area 0) and other connected areas. OSPF routers carry full topology information about the backbone and connectivity information about all of the areas. Within each area, OSPF routers exchange full topology information about that area because the boundaries of areas fall within a router. This results in router interfaces being bound to areas. Because a router has multiple interfaces, it can be in multiple areas as well. However, this design would require the router to run separate SPF calculations for each area; therefore, you should design the network appropriately.

Integrated IS-IS uses the same two levels of hierarchy as OSPF; however, the two protocols differ in the quantity of information that is carried inside each area. Within an area, integrated IS-IS routers send all traffic that needs to go out of the area to the nearest IS-IS Level 2 router. OSPF, on the other hand, injects all the connectivity information about the other areas into each area. This enables every router in an OSPF area to choose the optimal area border router (ABR) for traffic that needs to go out of its area.

IS-IS area boundaries are segmented on a link between two routers; therefore, a router is typically only in one area. However, it is hard to deploy true hierarchical network architecture with IS-IS for several reasons, one of which is that IS-IS operates best with very large areas with at least one router in an area for it to operate properly. This is one of the factors that causes ISPs to deploy a single IS-IS area as their core, although many ISPs are now reconsidering OSPF.