Virtual Links; Figure 173 Ospf: Types Of Routers; Table 77 Ospf: Redistribution From Other Sources To Each Type Of Area - ZyXEL Communications ZyWALL 1050 User Manual

ZyWALL 1050 User's Guide
• An Autonomous System Boundary Router (ASBR) exchanges routing information with
routers in networks outside the OSPF AS. This is called redistribution in OSPF.

Table 77 OSPF: Redistribution from Other Sources to Each Type of Area

SOURCE \ TYPE OF AREA
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 173 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.
260
NORMAL NSSA
Yes Yes
Yes Yes
STUB
No
Yes
Chapter 13 Routing Protocol