ZyXEL Communications ISG50 User Manual

Chapter 15 Routing Protocols

ISG50 User’s Guide

• 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.

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.

OSPF: Redistribution from Other Sources to Each Type of Area

Static routes

Yes

Yes

No

RIP

Yes

Yes

No