Designated router
A designated router (DR) is the router elected among all routers on a particular multiaccess network segment. The DR is elected based on the following default criteria:
If the priority setting on a OSPF router is set to 0, that means it can NEVER become a DR or BDR (Backup Designated Router).
When a DR fails and the BDR takes over, there is another election to see who becomes the replacement BDR.
The router sending the Hello packets with the highest priority.
If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID (Router ID) wins. NOTE: a RID is the highest logical (loopback) IP address configured on a router, if no logical/loopback IP address is set then the Router uses the highest IP address configured on its active interfaces. (e.g. 192.168.0.1 would be higher than 10.1.1.2).
Usually the router with the second highest priority number becomes the BDR.
The priority values range between 1 - 255, with a higher value increasing its chances of becoming DR or BDR.
IF a HIGHER priority OSPF router comes online AFTER the election has taken place, it will not become DR or BDR until (at least) the DR and BDR fail.
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. 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 Broadcast networks (Ethernet). 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.
Backup designated router
A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with second highest priority at the time of the last election.
