Subject: Re: Waiting State Question

From: John Smith <[email-masked]>
Date: Fri, 20 May 2005 03:12:16 +0100

Hi Mukul,

Extending the same logic cant we say that an if an interface comes out of waiting state
on recieving a Hello from a BDR then it may elect itself as the DR. Okay .. I think i
understand this.

A network will *always* have a DR even if its this single router. So we are sure that if
we recieve a HELLO from the BDR then it will *always* list the DR. SO we now know the
identity of the DR and the BDR.

But, if i recieve a HELLO from the DR cant i look at the BDR field and know that there
exists a BDR and declare that same router as the BDR? OR is the assumption that unless i
hear from that BDR i cannot confirm that its still up. Is this it?

Pretty confused,
John

Mukul Said :

My guess is that if an interface comes out of the waiting state on receiving a
Hello from DR (without having received a Hello from BDR), it may elect itself
as BDR. This way many routers may elect themselves as BDR. Now all these BDR
claimants (except one) will ultimately take their claims to BDRship back but in
the process each router on the LAN may have to do several DR elections.

Here is a paper we wrote recently that may shed further light on this:
http://cs.uwm.edu/~mukul/ospflan.pdf

Thanks,
Mukul

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From: Erblichs <[email-masked]>
Date: Thu, 19 May 2005 19:50:05 -0700

John Smith,

	First...
	No. A DR/BDR capable interface doesn't need
	to have a DR and/or a BDR.

	If a single router exists, it can configure
	itself not to elect itself as the DR. Normally
	setting its priority to a set value.

	Mitchell Erblich
	------------------------

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From: Mukul Goyal <[email-masked]>
Date: Fri, 20 May 2005 07:49:03 -0500

Hi John,

> on recieving a Hello from a BDR then it may elect itself as the DR. Okay .. I

Suppose router A comes out of waiting state on receiving BDRship claim from
router B. If router A has not received any claim to DRship yet, it will elect
router B as BDR (assuming B is the only claimant to BDRship) and DR as well (no
claimant for DRship; choose BDR as DR). So, router A can not elect itself as
DR.

> identity of the DR and the BDR.

Exactly. A router can elect itself as BDR only if there is some other router
that claims DRship.

> hear from that BDR i cannot confirm that its still up. Is this it?

Well, consider the scenario where router X comes up on the LAN at time 0, 
router Y comes up at time 20 and router Z (with higher Router ID than router Y)
comes up at time 40. At time 40, just after router Z comes up, router X comes
out of the waiting state, elects itself as DR and router Y as BDR. Router X
will not elect router Z as BDR since it has not yet established bidirectional
comunication with router Z. At this time, routers Y and Z are still in the
waiting state. Router Y will come out of waiting state at time 60 and would
have established bidirectional communication with router Z by this time. So, at
time 60, rouer Y will elect router Z as BDR (since router Z has a higher Router
ID than router Y). So, even though router X thinks (until it has established
bidirectional communication with router Z) that router Y is BDR, this is not
actually the case.

So, you are right, unless I hear a BDRship claim from a router itself, I can not
be sure that this router indeed claims BDRship.

Thanks,
Mukul