Subject: DR & BDR Election

From: wu min <[email-masked]>
Date: Thu, 23 May 2002 01:30:39 +0000

Hi,

I am pretty new to OSPF and I have a couple of questions. I would appreciate
some help.

1. Can a router declares itself as DR and BDR simultaneously (just in case
that it may not be choosen to be a DR, it have some chance to take up BDR
post)?

2. Should *other* routers keep declaring themselves (eligible to be) DR or
BDR in HelloPackets eventhough both DR and BDR were already choosen?

3. In section 9.4, page 76 of RFC2328, step (4), why does the newly selected
DR or BDR (or resigning DR or BDR), is required to run step (2) and (3)
again? Take for instance, if it knew that it was being elected as the new
designated DR or BDR in the first run of step (2) and (3), repeating step
(2) and step (3) again will never change its view. Am I missing something
here?

4. Is it true that if a router that was previously a BDR (and also has been
declaring itself as BDR only in its HelloPacket), but now being elected as
new DR to replace the old one, must declare itself as DS in its subsequence
HelloPackets?

Thanks in advance.

-Tze Ven

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From: Alex Zinin <[email-masked]>
Date: Wed, 22 May 2002 18:55:34 -0700

Tze Ven:

 A short answer to your questions would be---please follow
 the spec. However, I put explanations inline below.

> that it may not be choosen to be a DR, it have some chance to take up BDR

No, it cannot.
The router puts in the hello packets the DR and BDR it has locally
calculated by performing the algo in section 9.4, which ensures that
a router never selects itself as both the DR and the BDR.

> BDR in HelloPackets eventhough both DR and BDR were already choosen?

No, they should not. When a router elects the DR and the BDR, it
should start announcing _these_ values in its Hello packets.

> (2) and step (3) again will never change its view. Am I missing something

Please see the following URL for an answer:
http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind0105&L=OSPF&P=R7503

> new DR to replace the old one, must declare itself as DS in its subsequence

Please see my answers to your first two questions.

Alex

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From: Erblichs <[email-masked]>
Date: Thu, 23 May 2002 21:32:38 -0700

To add additional confusion...

In footnote RFC 2328[5], when a DR expires the BDR will become
both the DR and the BDR for a short period of time.

Wanted to clarify #1 below. Of course, a router may on one
interface be the DR and on another interface be the BDR. In
addition only after a wait period, can the DR or BDR be
elected. If a DR is elected and a router with a higher priority
comes up after the election. The election is not run again..

Mitchell Erblich