ZyXEL Communications ZyWall ATP series User Manual page 315

Chapter 10 Routing
Figure 213 OSPF: Types of Routers
In order to reduce the amount of traffic between routers, a group of routers that are directly connected
to each other selects a designated router (DR) and a backup designated router (BDR). All of the routers
only exchange information with the DR and the BDR, instead of exchanging information with all of the
other routers in the group. The DR and BDR are selected by priority; if two routers have the same priority,
the highest router ID is used.
The DR and BDR are selected in each group of routers that are directly connected to each other. If a
router is directly connected to several groups, it might be a DR in one group, a BDR in another group,
and neither in a third group all at the same time.
Virtual Links
In some OSPF AS, it is not possible for an area to be directly connected to the backbone. In this case,
you can create a virtual link through an intermediate area to logically connect the area to the
backbone. This is illustrated in the following example.
Figure 214 OSPF: Virtual Link
In this example, area 100 does not have a direct connection to the backbone. As a result, you should
set up a virtual link on both ABR in area 10. The virtual link becomes the connection between area 100
and the backbone.
You cannot create a virtual link to a router in a different area.
OSPF Configuration
Follow these steps when you configure OSPF on the Zyxel Device.
ZyWALL ATP Series User's Guide
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