To solve the problems associated with flat routing protocols, additional features are implemented in hierarchical routing protocols to support large networks—for example, some support an area-based design.
Hierarchical routing protocols are typically classless link-state protocols. Recall from Chapter 6 that classless means that routing updates include subnet masks in their routing updates; therefore, the routing protocol supports VLSM.
Hierarchy is part of the implementation of link-state protocols with the concept of backbone and nonbackbone areas. With link-state protocols such as OSPF and IS-IS, large networks are divided into multiple areas.
Route summarization can be performed manually in hierarchical protocols and is required in most cases. With the help of route summarization, smaller routing updates propagate among areas, resulting in higher scalability. Instabilities in one part of the network are isolated, and convergence is greatly improved. Summarization can be performed on an arbitrary bit boundary within an IP address. Note, however, that OSPF supports summarization on only specific routers called area border routers and autonomous system boundary routers.
Although it is a classless hybrid protocol, EIGRP is considered a flat routing protocol because it is not area-based. Because EIGRP also supports manual summarization, EIGRP can be used in a hierarchical network design by dividing the network into areas. A hierarchical design is not necessary in EIGRP, but one is recommended for large networks.
NOTE Although it too is classless and supports manual summarization, RIPv2 is considered a flat protocol. RIPv2 is not recommended for large networks because it is a distance vector protocol.
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