Page "Open Shortest Path First" Paragraph 66

DR's 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 or ATM.

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 LSDB Synch Simulation shows how DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.