Re: DR/BDR election in OSPF - Please Help!

Hello Binoy ,

check out these 2 links...they explain well

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.shtml

http://www.cisco.com/en/US/customer/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t21

----- Original Message -----
From: "Binoy Udayabhanu" <binoy_uday_at_yahoo.fr>
To: "CCIE Groupstudy" <ccielab_at_groupstudy.com>
Sent: Tuesday, September 08, 2009 6:49 AM
Subject: DR/BDR election in OSPF - Please Help!

> Hi experts,
>
> I am having some doubts in OSPF DR/BDR election. The following is the
> excerpt
> from Jeff Doyle regarding this subject:
>
> The election procedure of the DR and BDR is as follows:
> 1. After 2-Way communication has been established with one or more
> neighbors,
> examine the
> Priority, DR, and BDR fields of each neighbor's Hello. List all routers
> eligible for election (that is,
> routers with priority greater than 0 and whose neighbor state is at least
> 2-Way); all routers
> declaring themselves to be the DR (their own interface address is in the
> DR
> field of the Hello
> packet); and all routers declaring themselves to be the BDR (their own
> interface address is in the
> BDR field of the Hello packet). The calculating router will include itself
> on
> this list unless it is
> ineligible.
> 2. From the list of
> eligible routers, create a subset of all routers not
> claiming to be the DR (routers
> declaring themselves to be the DR cannot be elected BDR).
> 3. If one or more neighbors in this subset include its own interface
> address
> in the BDR field, the
> neighbor with the highest priority will be declared the BDR. In a tie, the
> neighbor with the highest
> Router ID will be chosen.
> 4. If no router in the subset claims to be the BDR, the neighbor with the
> highest priority will become
> the BDR. In a tie, the neighbor with the highest Router ID will be chosen.
> 5. If one or more of the eligible routers include their own address in the
> DR
> field, the neighbor with
> the highest priority will be declared the DR. In a tie, the neighbor with
> the
> highest Router ID will
> be chosen.
> 6. If no router has declared itself the DR, the newly elected BDR will
> become
> the DR.
> 7. If the router performing the calculation is the newly elected DR or
> BDR, or
> if it is
> no longer the DR or BDR, repeat steps 2 through 6.
>
> Please clarify what exactly is said in the No:7 statement. If the
> calculating
> router is the new DR, we can safely assume that there is a vacancy for BDR
> and
> we should repeat steps 2 to 6 so as to find a new BDR. But, what if it is
> the
> newly elected BDR, why do we need to do a repetition of the process. And,
> the
> second part: or if it is
> no longer the DR or BDR, I don't understand it at
> all. Please help. Also, why the BDR is elected before DR? Can someone
> please
> elaborate more than what is given in statement No:2.
>
> Thanks & Regards,
> Binoy
>
>
>
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Received on Tue Sep 08 2009 - 17:32:52 ART