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The Point is to Select one mutually agreed Router (and it's Backup) with whom All the Routers in that Multi Access Network communicates with for all the Updates they have to share with All Other. It's Like election of CR in a Class, Rather Everyone Talking (Generating Noise level) why not elect one who represent the All. and with whom All Corresponds. It's a vague definition. I hope it conveys the Point.
TheRouter withe the highest Router ID will be "DR" then the "BDR" well be the2nd Router, and you can change the order by changing the Priorty ID to give the Router you need to be "DR" the highest Priority.
1. If you configure manual router-id then:
Highest router ID will be DR then second highest router-id will be BDR
2. If you don't configure router-id then it will check loopback address.Then among all loopback addresses:
Highest Loopback Address will be set as router-id and among all routers highest router ID will be set as DR and second highest router ID will be set as BDR.
3. If there's no router-id set or there's no loppback interface present then:
Router will check physical interface that is up.Among all physical interfaces the highest address will be set as router-id.Then among all routers highest router-id will be set as DR and second highest ID will be set as BDR.
If you want to manually set any router as DR then:
set the highest ospf priority value for that router.
#ip ospf priority number-value
If you don't want a router to be a DR/BDR then:
set router ospf priority0
#ip ospf priority0
OSPF election process happens in the Init phase during wich OSPF Hello protocol (multicast) is in charge of the DR/BDR elections for a network segment combining various factors in the following order:
1- Choose the highest OSPF priority (default is1, note that if you configure priority of0 that router wont enter the election process - used on FR networks.)
2- Choose the highest router-id
3- Choose the highest loopback interface
4- Choose the highest configured physical interface (must be up/up)
Mind that there is no preemption meaning that when a DR goes ofline, the BDR takes his place and when it comes back online it wont be the DR again, for it to be considered for another election process in a network segment, all DR/BDR must go offline so that one router can become the DR again for that segment.
If we looking to how the OSPF working we can see when any change happened in the network every rourter will update his neigbours as the topology table must be the same on all routers which will make traffic storm .. so to solve this problem we elect DR and BDR and all router will establish a full relationship with them .. and the other routers will establish a2 way relationship , so when any change happened in the network topology the router with the network change will update the dr and the bdr then they will update all the adjacencies.
Designated Router (DR) have haigh value and Backup Designated Router (BDR) have low value and selection of BDR is depand of presence of DR
DR = designated router BDR = backup designated router Overall, the designated router controls the area its in. If it goes down, the BDR takes over as a fail-over. or redundency. BR (boarder router) and ASBR (AS Border Route)
DR is abbrivated as designated router where as BDR border designated router
router are made election between them and elected the highest id of router and made his as dr. which is having highest ID become the DR . ospf brought the area concept and in that area are divided by the each router . so that BDR for electing from border or back bone area .
If there are multiple router in multiaccess segment, then each router will forward the update to every other router. so router cpu utilization increases as again checking the routing table for the update link bandwidth will increase and bcoz of this each router will get the same update which was already in his routing table. To stop this unnecessary process, DR and BDR elecetion come in to multiaccess segment.
