OSPF - The DR and BDR Roles
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. Non-DR and non-BDR routers only exchange routing information with the DR and BDR, rather than exchanging updates with every other router upon the segment. This, in turn significantly reduces the amount of OSPF routing updates that need to be sent.
NOTE OSPF does not elect DR/BDR roles upon point-to-point links, i.e 2 directly connected routers. Further details on the different OSPF network types can be found 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 (/neighbor state) with the Designated and Backup Designated Routers. Non-DR and Non-BDR's on the other hand form the 2-way neighbor state. This means that they both send/receive each other's HELLO's, but no routing updates are exchanged between one another.