Today I am going to dive into OSPF router adjacency roles. In broadcast/non-broadcast multi-access network topologies, OSPF elects routers to maintain a certain role within the topology to provide redundancy, and make communication/updates more efficient. The roles that make OSPF broadcast network types run smoothly are the DR (Designated Router), BDR (Backup Designated Router), and DRother. All three of these play a crucial role when the OSPF network is fully converged.
DR- The DR is the primary router of the network topology. There is only one DR router per segment, and the DR is chosen by selecting the router with the highest priority or highest RID if there were to be a tie-breaker. Having a DR reduces the flooding of LSAs, specifically LSA type 2 packets. The DR is the only router on the topology that originates and floods LSA Type 2 (Network LSA) packets which describe all routers on that broadcast segment. The DR and BDR form full adjacencies with all routers on a network segment, DROthers also form full adjacencies but only with links connected to the DR and BDR. The DR also acts as the central point of the topology as well, whenever there is a change in topology by a DRother like a link going down, instead of flooding it the DRother sends a multicast to the address of 224.0.0.6 which sends traffic to only the DR and BDR. Once the DR receives this updated LSA, it then floods it to the multicast address of 224.0.0.5 which is the address for every OSPF router.
BDR- The BDR is the backup router ready to take over the DR in case it were to fail for whatever reason. The router that is chosen as the BDR during the election has the second highest priority or second highest RID if there was a tie breaker. The BDR election occurs at the same time as the DR during the two-way state, when the neighbors are certain to reach full adjacency.The BDR also maintains a full adjacency with every other router it is connected to. This allows the OSPF topology to be redundant as it will take over the DR immediately once it is detected to be down.
DROther- The last role for a router is the DROther, this is the role of routers in broadcast networks that were not selected to become the DR and BDR. These routers remain in the two-way state when connected to any other router except the DR and BDR. When the DROther is connected to the DR and BDR, then those links will reach a full adjacency. The DROthers do not generate or flood Type 2 LSAs, but receive and install them in their LSDB. They do however generate and flood Type 1 LSAs.
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