Router 2s Perspective

Each router's perspective of the network differs. Although it would be somewhat monotonous to examine each router's perspective, a second example may prove useful in demonstrating the impact that perspective has on the shortest-path tree. Figure 12-13 demonstrates the shortest-path tree for Router 2.

Figure 12-13: Router 2's shortest-path tree.

Table 12-3 provides the summarized routing costs to all known destinations from Router 2's perspective.

Table 12-3: Costs from Router 2 to Known Destinations

Destination

Hops Away

Cumulative Cost

193.1.2.0

0

193.1.5.0

1

64

193.1.6.0

1

1,768

193.1.6.0

2

74 (64 + 10)

193.1.1.0

1

1

193.1.3.0

2

65 (1 + 64)

193.1.4.0

2

65 (1 + 64)

Comparing Table 12-2 and Table 12-3 demonstrates that the cumulative distances between a source and destination in a network can vary based on the starting point. Perspective, it seems, is everything. This is why OSPF routers use data obtained from other routers via LSA updates to develop their own perspective of the network instead of directly updating their routing tables with that information.

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