?– Periodic refresh updates are more infrequent than for distance vector protocols.
?– Networks running link-state routing protocols can be segmented into area hierarchies,
limiting the scope of route changes.
?– Networks running link-state routing protocols support classless addressing.
?– Networks running link-state routing protocols support route summarization.
Link-state protocols use a two-layer network hierarchy, as shown in Figure 3-27.
Figure 3-27 Link-State Network Hierarchy
The two-layer network hierarchy contains two primary elements:
?– Area: An area is a grouping of networks. Areas are logical subdivisions of the autonomous
system (AS).
?– Autonomous system: An AS consists of a collection of networks under a common
administration that share a common routing strategy. An AS, sometimes called a domain, can
be logically subdivided into multiple areas.
Area 0
Autonomous System
Area 2 Area 1
Router A
Router B Router C
Router E Router D
118 Chapter 3: Medium-Sized Routed Network Construction
Within each AS, a contiguous backbone area must be de?¬?ned. All other nonbackbone areas are
connected off the backbone area. The backbone area is the transition area because all other areas
communicate through it.
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