Adjacencies on Multi-Access Networks
It isn't efficient for every router on a multi-access (broadcast or NBMA) network to build full adjacencies with every other router on that network. So, OSPF uses the concepts of designated routers (DRs) and backup designated routers (BDR) to reduce the number of adjacencies that must be built (and reduce the number of LSAs flooded throughout the area for the common network). Each router on the network will build a full adjacency with the DR and the BDR and leave all other neighbors on that network in the two-way state. The DR is responsible for advertising a link to the network and for flooding LSAs to other routers on the link.
The DR and BDR are elected based on the router priority (configured on a per ...
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