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Chapter 15 Routing Protocols
ISG50 User’s Guide
306
• An Area Border Router (ABR) connects two or more areas. It is a member of all the areas to
which it is connected, and it filters, summarizes, and exchanges routing information between
them.
• An Autonomous System Boundary Router (ASBR) exchanges routing information with routers in
networks outside the OSPF AS. This is called redistribution in OSPF.
• 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 202 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.
Table 93 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 No