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DR/BDR Election Process in Broadcast Network


I am tiring to understand OSPF DR/BDR election process in broadcast network. So I have check the behavior using following topology.


Topo 1.png

Now I am trying to restart all routers. Please note that I have filter R1 router packet to demonstrate. But all other Router Packet captures have same result.

As expected all routers advertise BDR/DR as during the waiting time.


All routers (except DR) sending the first Hello packet after waiting time , listing elected DR IP address in both DR and BDR field.

1st hello.png

Frist Hello packet send by the elected DR after the waiting interval has the correct BDR and DR field.

   DR Hello.png

After that all  other Routers list correct BDR and DR in Hello Packet.


Please help me to understand following observations.

 1. Why all Routers (except DR) list elected DR  IP in both BDR and DR field ? 

 2. Why only elected DR have correct DR and BDR fields in the first Hello after the waiting time ?


Thank you very much for your help ....


8 Replies 8

Richard Burts
VIP Community Legend VIP Community Legend
VIP Community Legend

I have not looked into this thoroughly and am not authoritative about it, but this is my understanding of the process. As the network is coming up there are no DR and no BDR. There is a waiting period. After the waiting period ends there is an election  and that election is for BDR (perhaps not obvious but is an important point). The appropriate router wins the election and becomes BDR (and at that point there still is no DR). So the BDR field of the Hello identifies that router. When there is a BDR and no DR then the BDR automatically becomes DR. So now the Hello packet identifies the router as DR. Then there is a new election for BDR and another router will win that election. But until that new election the BDR field is not updated and the router which won the first election is listed in both.



Hi @Richard Burts 

I also think electing BDR first and promote it for the DR will be the reason behind the duplication of BDR and DR. But confusion point is when I start DR  Router (Expected to be DR) slightly earlier than (2S) other Routers , first packet send by the DR after the waiting time has correct BDR and DR fields (without duplicating DR and BDR) and also first packet after the waiting time send by all other routers has  correct BDR and DR fields (without duplicating DR and BDR).

But if I delay the startup up of DR Router slightly (2S) than other router then first packet send by the other all routers has duplicated DR and BDR filed but first Hello packet send by the elected DR has correct DR and BDR field.

So my point is

  why all other Router waiting for the elected DR Hello (which has correct DR/BDR field) to elect BDR ?

  is it one of responsibilities assign for the elected DR to elect BDR and advertise for the other Routers ?

I could not find concept like that in the Cisco press please help me to understand this behavior clearly..


Elliot Dierksen
VIP Engager VIP Engager
VIP Engager

I don't know this for a fact, but I think that there is no concept of preemption in DR/BDR election. Once something has the role, it only relinquishes it if it goes away. That DR or BDR going away cause a new election.

I agree that there is no concept of preemption for OSPF election of DR/BDR. I have tested this and observed these results. Brought up a broadcast network with several OSPF routers, each with default priority. Waited for the election of DR and BDR. Brought up a new OSPF router in that network configured with a higher OSPF priority. Even with a router with higher priority present in the network there was no change in DR/BDR. Then rebooted the existing DR so it was no longer in service. The result is that the previous BDR became the new DR and the router with higher priority became new BDR. So there is no preemption and it would take 2 OSPF elections before the router with higher priority became DR.




Dear @Elliot Dierksen  and @Richard Burts ,


I also agreed there is no preemption after DR/BDR is elected. Here my issue is if we restart OSPF process of all connecting routers same time , all other routers are waited for the Hello packet from the elected DR (with correct BDR and DR) to elect BDR. Until they duplicate DR IP in the BDR .


But if we check debug in elected DR it is straight forward.


Please help me to understand the theory behind above observation.



Hello @Ariyarathna ,

the main take away is the non preemptive nature of DR and BDR in broadcast segments as noted by other colleagues.


You are looking for a so great level of detail including Hello contents in transient that are not so important as they can vary for a delay of 2 seconds .


In real world most of the times the DR is the device with the highest uptime on the segmenr and this can be an issue in some cases.


Also the election processes have some tolerance built in that is represented by the wait interval : all devices able to send the first Hello packet within that time can take part to the election.


As you have noted what happens during the transient is not documented not al the level of details of your analysis.


The way you make the tests influence the transient for sure: for example if you "reload" the switch in the middle connecting all devices instead of clearing the OSPF processes you may see something different.


Compare OSPF behavior with IS-IS where the DIS can be pre-empted and there is no backup DIS.


Hope to help



There are some things about the output that I do not understand. For example the router generating the output (R1?) has 7 reports of DR/BDR election. with time stamp of 








While the output from the other router reports only a single election. Is this testing using real hardware? In which case what model of router and what version of code? Or is this testing with some emulator?



Hi @Richard Burts ,

Sorry for the inconvenience caused. I did not post all debug output of the Elected DR router (As there is no change in elected DR/BD in  related to the elected DR Router).So I have done it again and attached all four router debugs herewith,


I have test this scenario with vIOL, vIOS and CSR (Everest) images in EVE platform and with C3640 in GNS3 platform.

I got same result for all images and platforms.




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