ZyXEL Communications ZYWALL USG 20 Manual page 318

Chapter 14 Routing Protocols
• An Autonomous System Boundary Router (ASBR) exchanges routing information
with routers in networks outside the OSPF AS. This is called redistribution in
OSPF.
Table 85 OSPF: Redistribution from Other Sources to Each Type of Area
SOURCE \ TYPE OF AREA NORMAL
Static routes
RIP
• 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 192 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
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NSSA
Yes Yes
Yes Yes
STUB
No
Yes
ZyWALL USG 20/20W User's Guide