On an multiaccess network OSPF prevents every router on the segment from becoming fully adjacent neighbors with each other through the use of OSPF Designated Routers (DR).

If OSPF did not use the concept of having a DR then each router on the multiaccess segment would directly share its LSDB with each other, we can calculate the number of instances of  LSDB flooding that would occur if there wasn’t a DR on a multiaccess segment with the full mesh formula which is N(N-1)/2.

So without OSPF DRs if we had 10 routers then where would be 45 LSDBs flooded between the adjacent OSPF routers (N(N1-1)/2 = 10(10-1)/2 = 45). This could potentially cause a serious amount of overhead due to the massive amount of redundant LSAs that would be propagating the network. This is the main reason why OSPF DRs are used, another good point about OSPF DRs is that they originate type 2 LSAs which represent a subnet. I will make a post on all of the LSA types soon.

OSPF will start to perform a DR election when it starts 2-way state on the first neighbor it sees on an interface if the hello received from the neighbor has a DR of 0.0.0.0 (0.0.0.0 as the DR in a hello packet means a DR has not yet been elected). OSPF will then wait the period specified in the Dead Timer, where it is waiting for other routers to communicate and come online. This timer is called the wait time and is used to allow other routers to come online and be active for the DR elections after a failure, if this did not occur then one of the first routers to be online would always be the DR.

If the received OSPF hello has a DR of anything else apart from 0.0.0.0 that indicates that DR elections have already taken place and OSPF will use the DR specified in the hello.

In each OSPF hello it sends the current DR and its Priority, if the DR priority of a received hello is higher than the current DR it recognises that as the current DR. The router priority is a value between 0 and 255 with the higher being better, the default is 1 and if it is set to 0 then the router will never become a DR. If there is a tie between OSPF priorities then the highest Router ID wins the tie breaker. The second most preferable DR becomes the Backup DR (BDR) and is used as a failover. Routers which are not the DR or BDR assume the DROther state. Once the DR election is complete the router will move into ExStart and start exchange Database Descriptors (DD).

All OSPF routers send their DDs to the multicast address of 224.0.0.6, this gets acknowledged by the DR sending the same packet back to the source as unicast. The DR then sends the DD it has received to all OSPF routers on the multi access segment via the multicast address of 224.0.0.5. This means that all routers on a multi access network to not need to constantly keep updating one another, they only have to update a central source and this is then sent to the segment.