696 | Open Shortest Path First (OSPFv2 and OSPFv3)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m Area Border Router (ABR)Within an AS, an Area Border (ABR) connects one or more areas to the Backbone. The ABR keeps a copyof the link-state database for every area it connects to, so it may keep multiple copies of the link statedatabase. An Area Border Router (ABR) takes information it has learned on one of its attached areas andcan summarize it before sending it out on other areas it is connected to.An ABR can connect to many areas in an AS, and is considered a member of each area it connects to.Autonomous System Border Router (ASBR)The Autonomous System Border Area Router (ASBR) connects to more than one AS, and exchangesinformation with the routers in other ASs. Generally the ASBR connects to a non-Interior Gate Protocol(IGP) such as BGP or uses static routes.Internal Router (IR)The Internal Router (IR) has adjacencies with ONLY routers in the same area, as Router E, M and I areshown in Figure 32-2.Designated and Backup Designated RoutersOSPF elects a Designated Router and a Backup Designated router. Among other things, thedesignated router is responsible for generating LSAs for the entire multiaccess network.Designated routers allow a reduction in network traffic and in the size of the topological database.• The Designated Router (DR) maintains a complete topology table of the network and sends theupdates to the other routers via multicast. All routers in an area form a slave/master relationship withthe DR. Every time a router sends an update, it sends it to the Designated Router (DR) and BackupDesignated Router (BDR). The DR sends the update out to all other routers in the area.• The Backup Designated Router (BDR) is the router that takes over if the DR fails.Each router exchanges information with the DR and BDR. The DR and BDR relay the information to theother routers. On broadcast network segments the number of OSPF packets is further reduced by the DRand BDR sending such OSPF updates to a multicast IP address that all OSPF routers on the networksegment are listening on.These router designations are not the same ad the router IDs discussed earlier. The Designated and BackupDesignated Routers are configurable in FTOS. If no DR or BDR is defined in FTOS, the system assignsthem. OSPF looks at the priority of the routers on the segment to determine which routers are the DR andBDR. The router with the highest priority is elected the DR. If there is a tie, then the router with the higherRouter ID takes precedence. After the DR is elected, the BDR is elected the same way. A router with arouter priority set to zero is cannot become the DR or BDR.
