Subject: [rbridge] Higher number is higher priority

From: Dinesh G Dutt <[email-masked]>
Date: Thu, 14 May 2009 17:01:37 -0700

All,

One another minor change. In spanning tree, lower number signifies 
higher priority whereas in IS-IS, higher number signifies higher 
priority (Radia tossed a coin with different results ? :). Given that 
Rbridges use IS-IS, I think we must follow the IS-IS logic and have 
higher numbers mean higher priority and not follow the .1Q bridges behavior.

In the base protocol spec, there are a few places where this needs to be 
fixed, if there is no objection. One place that jumps at me is the end 
of section 4.3 in draft12. This also needs to be fixed in the L2 IS-IS 
document too.

Dinesh

-- 
We make our world significant by the courage of our questions and by 
the depth of our answers.                               - Carl Sagan

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From: Radia Perlman <[email-masked]>
Date: Thu, 14 May 2009 18:30:29 -0700

Yes, I agree. We should switch TRILL to be numerically higher 
priority=higher priority.

And for historical interest -- I didn't flip coins. :-)

IS-IS was designed before spanning tree, and the choice of numerically 
higher=higher
priority was chosen because that's what any sane person would do.

But for spanning tree, it worked out really nicely to consider the fields
("root bridge priority", "root bridge ID", "cost to root", "transmitting 
bridge priority", "transmitting
bridge ID", "port ID")
as a multiprecision number where for comparing whether one BPDU is 
"better" than another.
Since "cost" obviously has to be "better" if it's smaller, it 
constrained all the other values
to be "smaller is better".

I wonder if it drives people managing bridges crazy to have to set the 
priority to be smaller
to make it higher priority?

Sorry about that...but it did make the description of "better Hello" 
really elegant.

Radia