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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
132 Switch Management and Operating Concepts
Router A
Router B
128.213.0.0
192.213.11.0
0
10
5
10
10
Router C
222.211.10.0
Figure 5-21. Constructing a Shortest Path Tree - Completed
Note that this shortest path tree is only from the viewpoint of
Router A. The cost of the link from Router B to Router A, for
instance is not important to constructing Router A’s shortest
path tree, but is very important when Router B is constructing
its shortest path tree.
Note also that directly connected networks are reached at a
cost of 0, while other networks are reached at the cost
calculated in the shortest path tree.
Router A can now build its routing table using the network
addresses and costs calculated in building the above shortest
path tree.
Areas and Border Routers
OSPF link-state updates are forwarded to other routers by
flooding to all routers on the network. OSPF uses the concept
of areas to define where on the network routers that need to