Subject: [rbridge] R-Bridge Routing Requirements draft is now availabl e

From: Eric.Gray at marconi.com (Gray, Eric)
Date: Tue, 25 Oct 2005 10:28:13 -0300

Harald,

	The difference between flooding and broadcast is not
really small, nor is it necessarily a nit.

	Flooding is a bridge-local phenomena associated with
bridge learning, and not with explicitly multicast/broadcast
traffic.  People frequently equate it with broadcast because 
that is the worst-case for consideration in most bridging 
concerns.

	Flooding stops at each point along the spanning tree
where the destination address has already been learned, or
has not yet been aged out.

	That behavior is substantially different from either
multicast or broadcast distribution.

	Also, we need to be careful when classifying one thing
as a "special case" of another.  Some people once classified
VPN services as a "special case" of traffic engineering.  It
is altogether too easy to fall into a mental snare that way.

	For example, in this case (considering broadcast, multi-
cast and flooding), broadcast is the simplest thing to do and
multicast and flooding are both different variations of it.
In each case (multicast verses flooding), there is a method
(or circumstance) under which propagation terminates early in
comparison with broadcast.  To do broadcast, you need neither
of these two complications; to do either multicast or flooding, 
you need not include the capability to do the other.

	Consequently, we would be better off thinking of flooding
and multicast as distinct "special cases" of broadcast.

--
Eric

--> -----Original Message-----
--> From: rbridge-bounces at postel.org 
--> [mailto:rbridge-bounces at postel.org]On
--> Behalf Of Harald Tveit Alvestrand
--> Sent: Monday, October 24, 2005 10:54 PM
--> To: Developing a hybrid router/bridge.
--> Subject: Re: [rbridge] R-Bridge Routing Requirements draft is now
--> availabl e
--> 
--> 
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Earlier, you wrote:

--On 24. oktober 2005 13:28 -0300 "Gray, Eric" <Eric.Gray at marconi.com> 
wrote:

> Harald,
>
> 	I am not sure that multicast distribution trees is going
> to be sufficiently unambiguous.  There will be both multicast
> and flooding to take into account and these are not the same.

I was thinking of broadcast ("flooding") as a special kind of 
multicast..... I don't know how, inside the rbridge cloud, distributing to 
the "all RBridges" multicast is different from distributing a broadcast. 
But that's a nit, and may be obvious once the drafts are out.

for the unicast stuff, I think it makes sense to abandon the "tree" term, 
just because using the term makes people think of the particular way in 
which the RSTP algorithm configures forwarding tables in bridges.....