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Example When You Should Use A DR


Topology Image

In the image above you will see an example in which it is appropriate to use a DR in a OSPF network.


OSPF dictates that a subnet either should or should not use a Designated Router (DR) and Backup Designated Rout (BDR) based on the OSPF interface type (also sometimes called the OSPF network type). Several OSPF interface types exist, but for the CCNA exams you should be aware of two main types: point-to-point and broadcast. (These types can be configured with the "ip ospf network type" command.) These OSPF interface types make a general reference to the type of data-link protocol used. The broadcast type is for use on data links that support broadcast frames, such as LANs. The image above shows an example of a LAN. The routers would have interfaces with a broadcast type and would elect a DR and BDR.







References

Odom, Wendell (2008) CCNA ICND2 Official Exam Certification Guide, Second Edition. Indianapolis: Cisco Press.