I have started this chapter saying that some students think that only one DR should be for the whole OSPF area, while I have said that this is not true and the DR’s are elected based on number of segments. In this LAB I will show you how this happens.
You can see, I have 3 routers connected to switches. Here I can conclude the following:
- R1 and R2 form 1 broadcast network (1 segment)
- R2 and R3 form 1 broadcast network (1 segment)
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As I have explained already, the DR/BDR are elected based on segments. That means if I enable OSPF on our scenario, we should have 2 DR’s and 2 BDR’s (one for each segment), even that all are inside area 0.
To win time, I have already enabled OSPF on all routers and have put the router ids as following:
- R1: 1.1.1.1
- R2: 2.2.2.2
- R3: 3.3.3.3
Let’s see what we have on R1:
Ok!!! R1 says that he is the BDR on the segment that he has with R2 and R2 is the DR because R2 has a highest router id.
What about R2? If we think of it logically, he should be the DR for his segment with R1 and he should be the BDR for his segment with R3 because R3 router id is higher than R2’s one. You got what I mean? Let’s verify that:
Look at this ???? – R2 is having 2 peers: R3 and R1.
With the peering to R3, R2 is the BDR and R3 is the DR.
While if we check with the 2nd peer, R2 is showing as a DR and R1 as the BDR.
In a result, we do have the following:
Segment 1: R1 and R2
- DR: R2 (because his router id is higher than the router id of R1)
- BDR: R1
Segment 2: R2 and R3
- DR: R3 (because his router id is higher than the router id of R2)
- BDR: R2
So you can see, we do have 2 DR’s in the same area, and that takes us back to what I have said that the DR election is not based on the area but based on the segment(s) in that area.
That’s all what I needed to explain in this chapter, I hope you enjoyed it and see you in the upcoming one.
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