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OPSF Behaviour

Hi All,

Going through the OPSF configuration guide and got confused about the DR and BDR election process on the broadcast multi-access environment.

Its says that an OSPF router first prepares a list of routers which claims to be the DR and also BDR. I know that the routers should have 2-way state and a priority greater than 0 to be the eligible for the process.

Why does OSPF first elects BDR rathe than electing DR ???? Is there any special thing about this behavior..

Why I cannot have DR elected first and then BDR ???? There is no DR premption in OSPF so it shouldnot matter.

What are the advantages of selecting a BDR and disadvantage of selecting the DR first.

If I have 5 routers, which router will prepare the DR and BDR eligible list.

When the routers send the Hello packet they have DR and DBR field set to and after 2 way state it looks for DR and BDR, how this process works...

Waiting for a response ....


-amit singh

Hall of Fame Super Gold

Re: OPSF Behaviour


My understanding of the answer to your question is like this: there needs to be logic in OSPF for promoting the BDR to become DR when the routers discover that there is no longer a DR. There needs to be logic in OSPF for electing a BDR when the routers discover that there is no longer a BDR.

Given that those two pieces of logic must exist in OSPF and the code to implement them exists, then in the case where there is no DR it is easier to use the existing peices than it would be to create new logic and new code for the case of no DR.

They certainly could have implemented the protocol with a specification to elect the DR first in the case when there is no DR and no BDR. And a separate specification in the case that there is no DR and there is a BDR. But I believe that it would have been more complicated. It is more simple to have a consistent process:

- if there is no DR then promote the BDR

- if there is no BDR then elect the BDR.




Re: OPSF Behaviour

This is correct.... Rather than electing a DR, then repeating the election for the BDR, it's simpler to present logic for BDR election, only, and just promote the BDR to DR whenever we lose the DR. It makes the code simpler--and one thing you'll find about OSPF is that there's a lot of stuff in there just to make the code, rather than the protocol, simpler.



Community Member

Re: OPSF Behaviour

I think, at initially, if elec DR first, there is no way to change the DR until DR go down. Maybe there is a router not send hello at the same time that electing DR(in case elec DR first) (it is checking connected network or... :D ) so OSPF elec BDR first.

This is my trought :D

Re: OPSF Behaviour

Thanks for your reply First of all.

" I think, at initially, if elec DR first, there is no way to change the DR until DR go down "...........

This applies to the process of BDR election first. There is not DR preemtion so you cannot change the DR even if you change the proprity on some other router.So untill DR goes down you cannot change it.


-amit singh

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