OSPF DR/BDR ELECTION

[smvitayush1]

Hello All,

My doubt is in regarding the selection of DR/BDR in OSPF.

In the above scenario:

1. Lets say R0 with router id:10.10.10.1 is configured first followed R1 with router id: 10.10.10.2.

2. The interfaces are in Area 0.

Question: Why does R0 is selected as DR and R1 as BDR. Is it because OSPF process was configured first in R0?

Even though lets say R0 was configured first then followed by R1. But unless R1 was configured R0 did not have any router to start OSPF negotiation. When R1 was configured, since R1's router-id is greater than R0's, so why didn't R1 was selected as DR?

[balaji.bandi]

OSPF Process for election
a. Choose the highest OSPF priority (default is 1)
b. Choose the highest router-id
c. Choose the highest loopback interface
d. Choose the highest configured physical interface.

Step 1 . A router starting the ospf process and listen for ospf hellos, if none are heard with in the dead time, it declare itself the DR.
step 2 . If hellos from other routers are heard , the router with the highest ospf priority is elected DR, and election process start again for BDR.
A priority of zero removes a router from the election.
step 3. If two or more routers have the same ospf priority , the router with the highest router-id is elected DR, and the election process start again for BDR.
After a DR is elected , election do not take place again unless the DR or BDR are lost. default priority is 1.

in your case - If you can post the configuration and ospf output to see why your case other router elected as DR.

[smvitayush1]

Hi BB, thanks for the response. I think I understood the above scenario (as in why R0 was selected as DR).

1. As I said R0 was configured first, so it starts sending OSPF Hello on its interface gi0/0 as it is up/up.
2. Since R1 is not yet configured, so it doesn't hear back R1's OSPF Hello and the dead interval runs out. Hence R0 elects itself as DR.
3. Now when R1 is configured, R0 sends the OSPF Hello. Since the R0 has already elected itself as DR, its OSPF Hello contains contains that parameter.
4. Hence R0 is elected as DR, for the above scenario.

Thanks BB.
Regards,
Ayush