D-Link DES-3350SR User Manual page 112

The diagram above shows the network from the viewpoint of Router A. Router A can reach 192.213.11.0 through Router B with a
cost of 10 + 5 = 15. Router A can reach 222.211.10.0 through Router C with a cost of 10 + 10 = 20. Router A can also reach
222.211.10.0 through Router B and Router D with a cost of 10 + 5 + 10 = 25, but the cost is higher than the route through Router
C. This higher-cost route will not be included in the Router A's shortest path tree. The resulting tree will look like this:
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 zero, 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).
D-Link DES-3350SR Standalone Layer 3 Switch
128.213.0.0
5
192.213.11.0
Figure 9 - 14. Constructing a Shortest Path Tree
128.213.0.0
192.213.11.0
Figure 9 - 15. Constructing a Shortest Path Tree – Completed
Router A
0
10
Router B
5
Router D
10
222.211.10.0
Router A
0
10
Router B
5
102
10
Router C
10
10
Router C
10
222.211.10.0