OSPF routing

[v4l3ntin]

Hello everyone, I have a question in one exam which is: Refer to the exhibit. Which three statements describe the results of the OSPF election process of the topology that is shown in the exhibit? (Choose three.)

I have attached the picture form the exibit.

My anwers were: R2 will be elected DR., R1 will be elected BDR.( because has manually configure router-id), The R4 router ID is 172.16.1.1.

Cisco's answer were slightly different: R2 will be elected DR, R3 will be elected BDR because it has the numerically highest router-ID of 192.168.1.4 ( but has router-id not configured, you can see int he attached exibit) and The R4 router ID is 172.16.1.1.

Am I wrong with my answer that R1 will be elected bdr because has a manually configure router-id rahter than a bigger IP address ?

I feel confused and that's why I am asking here .

[M02@rt37]

Hello @v4l3ntin 

In OSPF, the election of the DR and BDR is based solely on the numerical value of the Router id (RID), regardless of whether it was manually configured or automatically chosen... The router with the highest RID become DR, and the one with the second highest become the BDR.

A common misconception is that a manually configured RID has a higher priority than an automatically assigned one, but this is not true during the election process—OSPF does not distinguish how the RID was assigned; it only compares the values.

In your exam, you selected R1 as BDR because it had a manually configured RID, but Cisco’s correct answer selected R3 as BDR due to having a higher RID value, even though it wasn’t manually set. 

[Joseph W. Doherty · Accepted solution]

Not germane to this specific case, but did want to mention in passing, that DR/BDR election is also controlled by manual configuration of IP OSPF priority, which takes precedence over RID, and the election isn't preemptive (i.e. a router coming on-line with better election criteria will not preempt an existing DR/BDR).