ZyXEL Communications ZyWall 110 User Manual page 279

Chapter 10 Routing
• A backbone router (BR) has at least one interface with area 0. By default, every router in area 0
is a backbone router, and so is every ABR.
Each type of router is illustrated in the following example.
Figure 189 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 190 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.
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