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 shownin the previous example.Designated and Backup Designated RoutersOSPF elects a designated router (DR) and a backup designated router (BDR). Among other things, the DRis 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 DR maintains a complete topology table of the network and sends the updates to the otherrouters via multicast. All routers in an area form a slave/master relationship with the DR. Every time arouter sends an update, the router sends it to the DR and BDR. The DR sends the update out to allother routers in the area.• The 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 described earlier. The DRs and BDRs areconfigurable in Dell Networking OS. If you do not define DR or BDR in Dell Networking OS, the systemassigns them. OSPF looks at the priority of the routers on the segment to determine which routers are theDR and BDR. The router with the highest priority is elected the DR. If there is a tie, the router with thehigher router ID takes precedence. After the DR is elected, the BDR is elected the same way. A router witha router priority set to zero cannot become the DR or BDR.Link-State Advertisements (LSAs)A link-state advertisement (LSA) communicates the router’s local routing topology to all other localrouters in the same area.The LSA types supported by Dell Networking are defined as follows:• Type 1: Router LSA — The router lists links to other routers or networks in the same area. Type 1 LSAsare flooded across their own area only. The link-state ID of the Type 1 LSA is the originating router ID.• Type 2: Network LSA — The DR in an area lists which routers are joined within the area. Type 2 LSAsare flooded across their own area only. The link-state ID of the Type 2 LSA is the IP interface addressof the DR.• Type 3: Summary LSA (OSPFv2), Inter-Area-Prefix LSA (OSPFv3) — An ABR takes information it haslearned on one of its attached areas and can summarize it before sending it out on other areas it isconnected to. The link-state ID of the Type 3 LSA is the destination network number.• Type 4: AS Border Router Summary LSA (OSPFv2), Inter-Area-Router LSA (OSPFv3) — In some cases,Type 5 External LSAs are flooded to areas where the detailed next-hop information may not be554 Open Shortest Path First (OSPFv2 and OSPFv3)
