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Webcast-Catalyst9k
New Member

OSPF DR BDR

1. In OSPF first BDR election takes place ?

2. How will a router decide whether to declare itself as DR or BDR? This question is in reference to RFC 2328 <<If no routers have declared themselves Backup Designated Router, choose the router having highest Router Priority, (again excluding those routers who have declared themselves Designated Router), and again use the Router ID to break tie >>>

Plz explain..

Thx

G

5 REPLIES
Bronze

Re: OSPF DR BDR

Hi

1) No, first elected DR then Second BDR.

2) basically, let say three routes are conected via ring, all of them are start and then election process also start as per OSPF algorithm, a router who has highet router ID or loopback , become DR, and second Highest ID is become BDR.

-Minu

Re: OSPF DR BDR

Hello,

in addition to Minu's post, have a look at this explanation from a previous post as well:

The OSPF protocol provides for automatic election of Designated Router (DR) and Backup Designated Router (BDR) on Broadcast and NBMA segments. There is no DR/BDR election on Point-to-Point and Point-to-Multipoint networks.

The OSPF interface priority is the parameter that controls DR election. Its default value is 1. When OSPF interface priority is 0, the router is not eligible to become a DR. If a router comes up on a network segment and there are no other routers there already, it will become a DR. If there is an existing DR (or BDR), the new router will NOT attempt to preempt them. If several routers come up at roughly the same time (or the DR fails), a new DR election process will be triggered. The router with the highest priority value then will become a DR.

In short, the DR/BDR election process is not deterministic and depends on the sequence of events. Therefore it is important to be able to prevent routers from EVER becoming DR/BDR, when that is appropriate for the topology in which they are connected. This is accomplished by setting their OSPF interface priority to 0:

ospf set interface priority 0

Regards,

GNT

Re: OSPF DR BDR

hi...

actully i also had confusion regarding to the election process... but before few months i put that point in discussion with net pro and i think some one is replied with in OSPF BDR is elected first and then DR is elected and when i asked why so... then acording to experts they replied me its just because of coding for the protocol... in order to reduce the complexity of coding, they programmed such.

hope this helps you

rate this post if it helps

regards

Devang

New Member

Re: OSPF DR BDR

I checked it with RFC 2328, BDR election happens first..

What I could not understand is how will the router decide whether it wants to become DR or BDR....

Any router which says it self as DR during election cannot be made BDR if it looses election..

Thx,

G

Hall of Fame Super Gold

Re: OSPF DR BDR

Gaurav

I do not understand your statement: Any router which says it self as DR during election cannot be made BDR if it looses election. Perhaps you can clarify what you meant.

As has been stated the election of BDR happens first. Election of the DR is by promoting the existing BDR to be DR followed by election of a new BDR.

Identification of DR and BDR is carried in the OSPF HELLO packet. If the HELLO has blank in the field for BDR then there is an election. In the election for BDR any router which is not already the DR and which has priority greater than zero asserts that it is a candidate in the election. Winning the election is based on difference in priority and if the best priority is equal for two or more routers then election is based on highest Router ID of those routers with the best priority.

HTH

Rick

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