Subject: [rbridge] Optional pseudonode

From: Radia Perlman <[email-masked]>
Date: Wed, 09 Jul 2008 10:49:47 -0700

The notion of automatic pseudonode minimization was apparently discussed 
at the last IETF in either rtgwg or IS-IS. I wasn't
there, but heard there was not much interest, because there is already a 
solution to that (configuration).

The reason it was brought up outside of TRILL was because some people 
thought it was a good enough idea to
deploy in IS-IS in general, an perhaps even OSPF.

However, I think that even if IS-IS and OSPF are not interested in 
retrofiting this feature, I think it is a good idea
to do it in TRILL. It is quite simple, safe, and does not require 
retrofit, and is actually more useful of a feature in
TRILL than in layer 3 deployments of IS-IS.

In a lot of TRILL deployments all the links will be pt-to-pt (bridges 
and shared media would have gone away), so it does
seem like a good idea to have a safe zero-configuration method of 
eliminating pseudonodes in that case (if all RBridge adjacencies
resulted in pseudonodes there would be 50% more LSPs). It's slightly 
more difficult to add it into the layer 3 IS-IS at
this point because there has to be a method of signalling ability to do 
this. But if we put it into the base TRILL spec now,
then all RBridges would support this.

I'd suggest doing it like this:

In the Hello, say that having the DRB specify the pseudonode ID as 7 
bytes of 0 means "just report all your adjacencies
on this cloud as pt-to-pt links". So it's completely up to the DRB (no 
potential mismatch of configurations) as to
whether the cloud should be reported as pt-to-pt or with a pseudonode.

When the DRB decides to use a zero for pseudonode does not need to be 
standardized, though we could give hints.
It could be configured ("if you are DRB, always use a pseudonode", or 
"never use a pseudonode").
Or it could use an algorithm such as "if  you have only 1 RBridge 
adjacency, don't use a pseudonode, but otherwise
use a pseudonode". or "if you have 2 or more RBridge adjacencies, or had 
2 or more adjacencies within the last hour, use
a pseudonode, otherwise don't".

And also mention that this does not change any behavior such as how LSPs 
are flooded on the link. This only affects what
LSPs are generated. And perhaps give an example. If R1 and R2 are the 
only RBridges on the link, then with
a pseudonode there are 3 LSPs: say R1.25 (the pseudonode), R1, and R2, 
where R1.25 reports connectivity to R1 and R2,
and R1 and R2 each just say they are connected to R1.25.
Whereas if DRB R1 declares no pseudonode, then there will be 2 LSPs: R1 
and R2 each reporting connectivity to each other.

Radia

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From: James Carlson <[email-masked]>
Date: Wed, 9 Jul 2008 14:29:35 -0400

> solution to that (configuration).

Not just that there was a solution, but what I heard was that it
"didn't matter."  In other words, if you configure everything as
multiple-access and needing a pseudo-node, who cares?  You get a small
increase in the number of LSPs, modern CPUs are fast, added complexity
(of any kind) is a bad thing, and so on.

> and shared media would have gone away), so it does

Exactly; that's why it's so much more interesting for us than for the
general case, and probably why it won't necessarily get traction
elsewhere.

> a pseudonode, otherwise don't".

I'd be in favor of the second suggested implementation -- it's simple
enough -- but obviously if one node makes the decision, it's no longer
a compatibility issue.

-- 
James Carlson, Solaris Networking              <[email-masked]>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677
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From: "Thomas, Matthew R" <[email-masked]>
Date: Wed, 9 Jul 2008 20:54:05 +0100

Hi Radia,
 
I have kept quiet on this one lately, however during the ISIS meeting I did reference my maths that was submitted for publication. Academia takes an age to publish. This was accepted for publication and for completeness can be found here :o)
 
http://dces.essex.ac.uk/staff/hunter/Thomas%20CommsLett%202008.pdf
 
Regards,
 
MT

________________________________

From: [email-masked] on behalf of Radia Perlman
Sent: Wed 09/07/2008 18:49
To: [email-masked] >> "Developing a hybrid router/bridge."
Subject: [rbridge] Optional pseudonode

Radia