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 - 09:19:27 ART