Internal Area Routers

Internal area routers must exchange LSAs directly with every other router in its area. This includes every internal area router as well as any border area routers that may also be members in its area. Figure 12-6 demonstrates the forwarding, or flooding, of LSAs throughout Area 1 of the sample OSPF network presented in this chapter's previous illustrations. It is important to note that same-area OSPF routers needn't be directly connected to each other to share LSA information. An OSPF router directly addresses LSA packets to every known router in its area and forwards those packets using any available links.

Figure 12-6: LSA flooding within Area 1.

Figure 12-6: LSA flooding within Area 1.

A subtle implication of Figure 12-6 is that convergence can occur quite rapidly. There are two reasons for this. The first is that an OSPF router directly addresses and transmits LSAs to all routers in its area simultaneously (known as flooding). This is in stark contrast to the neighbor-by-neighbor approach used by RIP to drive convergence. The result is an almost instantaneous convergence on a new topology within that area.

Convergence is also expedited through the definition and use of areas. Topological data is not propagated beyond an area's borders. Therefore, convergence needn't occur among all routers in the autonomous system, just the routers in the impacted area. This feature both expedites convergence and enhances the stability of the network because only a subset of the routers in the autonomous system experiences the instability that is innate in convergence.

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