Within OSPF, the role of the Designated Router (DR) and a Backup Designated Router (BDR) is to act as a central point for exchanging of OSPF information between multiple routers on the same, multiaccess broadcast network segment. Each non-DR and non-BDR router only exchanges routing information with the DR and BDR, instead of the exchanging updates with every router on the segment. This significantly reduces the amount of OSPF routing updates sent across the network.
Note: OSPF does not elect DR/BDR roles upon point-to-point links, i.e. two directly connected routers. You can find further details on the different OSPF network types at : https://www.ccexpert.us/ospf-network/different-network-types-and-ospf.html.
Figure 1 - No DR/BDR
Figure 2 - With Elected DR
Upon the segment, each router will go through an election process, to elect a DR and BDR. There are two rules used to determine who is elected:
- Priority - Router with the highest wins the election. The default priority is 1. This is configured on a per-interface level.
- Router ID - If there is a tie, the highest router ID wins the election.
Each router forms a full relationship (/neighbour state) with the Designated and Backup Designated Routers. Non-DR and Non-BDR’s on the other hand, form the 2-way neighbour state. This means that they both send/receive each other's HELLO’s, but no routing updates are exchanged between each other.