3.6 OSPF [v2 and v3]

A designated router (DR) is the router interface elected among all routers on a particular multi-access network segment, generally assumed to be broadcast multi-access. The basic neighbor discovery process (Hello), flooding (224.0.0.6), DR election (priority, RID). Special techniques, often vendor-dependent, may be needed to support the DR function on non-broadcast multi-access (NBMA) media. It is usually wise to configure the individual virtual circuits of a NBMA subnet as individual point-to-point lines.

DRs exist for the purpose of reducing network traffic by providing a source for routing updates. The DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in a multi-access network segment will form a slave/ master relationship with the DR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address 224.0.0.6. The DR will then send the update out to all other routers in the area, to the multicast address 224.0.0.5. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load . DRs and BDRs are always setup/ elected on OSPF broadcast networks. DR’s can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay. DRs or BDRs are not elected on point-to-point links (such as a point -to-point WAN connection) because the two routers on either sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3335-3344).  . Kindle Edition.

https://www.ietf.org/rfc/rfc2328.txt