ZyXEL ZyWALL USG 50 User Manual - Page 302
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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 83 OSPF: Redistribution from Other Sources to Each Type of Area SOURCE \ TYPE OF AREA NORMAL NSSA STUB Static routes Yes Yes No RIP Yes Yes Yes • 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 182 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 302 ZyWALL USG 50 User's Guide
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