Subject: Re: DR & BDR Election

From: wu min <[email-masked]>
Date: Thu, 23 May 2002 14:54:09 +0000

Hi Alex,

Thank you for the reply and thanks for pointing out the previous question
raised too.

>  A short answer to your questions would be---please follow
>  the spec.

Yes, sir!

For question (1) and (2), I am now clear of what DR and BDR fields in Hello
packets should be. I got them mixed up with priority field :)

However, I am still pretty vague with the election process. The following is
a scenario and the effects and actions taken by routers (this is from my
current understanding).

Lets say we have 4 routers, namely, S, T, U, and V with priority 2, 3, 4,
and 5 respectively. Initially none of the routers had interact with one
another yet. When all first broadcasted their respective Hello
packets, they declared themselves individually as DR but let BDR = 0.0.0.0.

At this stage, all routers have received this information and they start to
calculate (since this information is new to them, it triggers
NeighbourChange event, and thus the calculation) and found out that there is
no BDR yet and V should be nominated as DR.

V, the newly elected DR, repeats step (2) and (3), but its perception about
DR & BDR stays the same (DR = V; BDR= 0.0.0.0).

The next moment, all broadcast individual Hello packets again, each Hello
packet, except from U, still contain DR = V and BDR = 0.0.0.0. Hello packet
from U now contains DR = V and BDR = U. All routers see the change and begin
calculating again. Now every router realizes that BDR = U and DR = V and the
election process complete for all routers except router U.

U, the newly elected BDR, repeats step (2) and (3), but its perception about
DR & BDR stays the same (DR = V; BDR= U). Election process ends. Subsequent
Hello packets from all routers will have DR = V and BDR = U.

I believe some part is wrong, but please point out to me.

Further question: Should a router immediately broadcast a Hello packet when
its perception about DR/BDR changed? Or just wait for the Hello Timer to
expire?

-Tze Ven

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From: Yasuhiro Ohara <[email-masked]>
Date: Fri, 24 May 2002 11:16:07 +0900

> I believe some part is wrong, but please point out to me.

Why (and where) do you believe some part is wrong ? Practically It
will be more complexed depending when to receive the other's Hello
packets, but your understanding seems almost correct to me.

> its perception about DR/BDR changed? Or just wait for the Hello Timer to

It's implementation dependent, but I believe latter. If all the router
on a segment behaves like former, the segment's Hello will be almost
synchronized.

regards.
yasu.

---

From: Alex Zinin <[email-masked]>
Date: Thu, 23 May 2002 19:43:06 -0700

Tze Ven:

...

> no BDR yet and V should be nominated as DR.

Here, they _will_ elect the BDR as well. Please see section 9.4, step 2.

...

> its perception about DR/BDR changed? Or just wait for the Hello Timer to

Yasu is correct. You should just use the timer.

Alex

---

From: Yasuhiro Ohara <[email-masked]>
Date: Fri, 24 May 2002 12:18:18 +0900

zinin> > At this stage, all routers have received this information and
zinin> > they start to calculate (since this information is new to
zinin> > them, it triggers NeighbourChange event, and thus the
zinin> > calculation) and found out that there is no BDR yet and V
zinin> > should be nominated as DR.
zinin>
zinin> Here, they _will_ elect the BDR as well. Please see section
zinin> 9.4, step 2....

Strictly, guessing his assumption, at this time all the routers still
electing itself as DR, so the BDR will not be found except U.

regards
yasu.