Figure 156 Ospf: Types Of Routers; Figure 157 Ospf: Virtual Link; Virtual Links; Ospf Configuration - ZyXEL Communications Unified Security Gateway ZyWALL 300 User Manual

Chapter 13 Routing Protocols

Figure 156 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.

13.3.3 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 157 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.

13.3.4 OSPF Configuration

Follow these steps when you configure OSPF on the ZyWALL.
1 Enable OSPF.
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