Answer A is settings router 3 to have priority 0(0 means they are not allowed to participate in DR/BDR election)
Answer C is making R1 to have a higher priority(default is 1 but putting very high number is only for safety reasons to ensure that the R1 is elected DR)
You can use the command Router(config-if)#ip ospf priority 200
to change the priority to 200 (The default value is 1) and then you use
Router# clear ip ospf process
command to enforce your changes...
even more then this , if you don't clear the IP OSPF process if the DR fails the new DR will be the BDR (router 2 ) and R1 will become the new BDR. It does not take into account priority even if we have it set at 200 . The answers given are incomplete.
Since R2 is the BDR no matter which priority value is set for R1, even if it would be 200 and you applied clear ip ospf process command on R3, the new DR will be R2 since it was BDR and the election will be held for new BDR in this case it is R1 as the new BDR. by the way there are not correct answers but the only reasonable left options are A AND C
Option A alone makes R2 the DR and R1 the BDR
Option C alone makes R1 DR and R3 the BDR
when applied together R1 becomes DR and R2 BDR since it has higher ospf priority than R3
I would exclude option B since it is not mandatory to set the router-id manually.
The router-id is set automatically using the loopback interface with the highest IP, if there's no loopback interface then we use the interface with the highest IP. You can also set it manually.
When selecting DR, BDR, and DROTHER we look first at the OSPF priority (more priority value translates to higher priority in being selected as the DR.) If 2 or more routers have the same priority, we look for the highest router-id.
B and C are correct
router-id 192.168.100.1 for R1 has not been set
R1 priority to 200 will make it DR
setting a priority to 0 will not allowed to participate in DR/BDR election
If Router ID is not manually configured, the highest IP address on any of the loopback interfaces is selected as Router ID.If there are no loopback interfaces the highest IP address given to any active interface is selected as router ID. You don't have to manually set it.
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