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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Router A
Router B
Router C
128.213.0.0
192.213.11.0
222.211.10.0
0
10
10
5
10
Figure 5-24. 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 receive particular link-
state updates are located. This helps ensure that routing updates are not flooded throughout the entire
network and to reduce the amount of bandwidth consumed by updating the various router’s routing
tables.
Areas establish boundaries beyond which link-state updates do not need to be flooded. So the exchange
of link-state updates and the calculation of the shortest path tree are limited to the area that the router
is connected to.
Routers that have connections to more than one area are called Border Routers (BR). The Border
Routers have the responsibility of distributing necessary routing information and changes between
areas.
Areas are specific to the router interface. A router that has all of its interfaces in the same area is called
an Internal Router. A router that has interfaces in multiple areas is called a Border Router. Routers
that act as gateways to other networks (possibly using other routing protocols) are called Autonomous
System Border Routers (ASBRs).
Link-State Packets
There are different types of link-state packets, four are illustrated below:
• Router Link-State Updates − these describe a router’s links to destinations within an
area.
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