Re: [pim] BSR update

[John Zwiebel <jzwiebel at cisco.com>]

On Apr 11, 2006, at 11:32 AM, Venu Hemige wrote:

> John,
>
> > Yes, IFF one has not configured his network for redundancy.
> > If I sound like a broken record, sorry.  I just want to be sure
> > that forwarding a triggered BSM is not an option.  The cases it
> > fixes are few while the potential for causing problems that we
> > don't currently understand are many.
>
> I agree the cases fixed by forwarding a triggered BSM are few. I also
> agree that in a network configured for redundancy AND where there are
> not multiple routers rebooting, there should be no issue with not
> forwarding a triggered BSM. But I am yet to hear a case where it
> actually causes a problem.

The DNF-bit also means "trust me" but more importantly also means
"don't do an RPF-check" (that's why it means "trust me").  The need
to not do the RPF-check is the same as the problem one sees with
ARP.  The triggered BSM arrives well before ARP is determined
and well before the route to the RP is determined.

If this triggered BSM is forwarded, one might consider removing the
DNF-bit, to what effect?  To spread further downstream information
that should already be known in a "well-constructed network"?  (Which
should also be thought of as a network with a large-diameter)

I would like to think that Stig's example could only happen in a
network that doesn't have a lot of redundancy.  (Am I misinformed?)

Is this a good enough reason?

> I believe the goal
> should be to get the entire network to a consistent RP-SET as soon as
> possible.

I completely agree.  But, IMHO, ASAP can mean 60 seconds, the next
periodic BSM.  In a "well-constructed network" with a lot of redundancy
there shouldn't be a need to forward triggered BSMs.

If this isn't a good enough reason, please consider that the idea of
a unicast BSM seemed to be brilliant until faced with the reality
of implementation.  (I thought so and was surprised that it didn't
work.)

So, the "worst case" scenario I can come up with.  If we choose to
forward triggered BSMs, we could end up with BSMs looping throughout
the network in a BSM storm.  Not that I think it -will- happen, but
I do believe it a possibility.

Weighing that possibility against the desired reliability, I'd much
prefer to not forward triggered BSMs. 
 

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