Area Border Routers

Area border routers are responsible for maintaining topology information in their databases for each of the areas to which they contain interfaces. Therefore, if an area border router interconnects two different areas, it must exchange LSAs with peers in both networks. As with internal area routers, these LSAs are addressed and transmitted directly to its peers in each area. Figure 12-7 illustrates this.

Figure 12-7: Intra-area LSA flooding in an OSPF network by area border routers.

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Figure 12-7: Intra-area LSA flooding in an OSPF network by area border routers.

Another of the performance-enhancing features of OSPF is route summarization. Topological information about an area is not shared with other routers outside that area. Instead, the area border router summarizes all the addresses contained in all the areas to which it is connected. This summarized routing data is then shared, via Type 3 LSAs, with peer routers in each of the areas it interconnects.

In Figure 12-7, the border area router advertises this summarized data directly to all routers in Area 0. OSPF prevents areas numbered 1 from directly connecting to each other. All such inter- connections must occur via Area 0. Therefore, it is implied that border area routers interconnect Area 0 with at least one nonzero numbered area.

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