190 Chapter 7: Using OSPF Across Multiple Areas
Figure 7-3 The Propagation of LSAs
OSPF Path Selection Between Areas
The local routing table on a router depends on its position in the network and the type of area it is
in. If there are routes with different routing information sources to the same destination, the router
chooses the path with the lowest administrative distance. If both are OSPF, OSPF will select lower
type advertisements ﬁrst and choose lower costs to break ties. OSPF, like all IP routing protocols
on Cisco IOS, is capable of load-balancing and will automatically distribute the load over four
Remember the sequence of events:
1. The router receives LSAs.
2. The router builds the topological database.
3. The router runs the Dijkstra algorithm, from which the shortest path is chosen and entered into
the routing table.
Thus, the routing table is the conclusion of the decision-making process. The routing table
displays information on how that decision was made by including the metric for each route. This
enables you to view the operation of the network.
External & Area 4
Area 3 LSAs