Subject: [rbridge] Granularity of Tree Root Priority

From: Donald Eastlake <[email-masked]>
Date: Wed, 23 Sep 2009 00:15:14 -0400

Currently, an RBridge has a single priority for its nickname or
nicknames to be distribution tree root(s). This made lots of sense
when each RBridge could only have one nickname.

Now that selected RBridges can be configured to have more than one
nickname, I think it would make sense to have this priority be per
nickname. That way you can get the current effect by configuring them
all for a particular RBridge to be the same priority, if that is what
you want. But you can also configure to different priorities the
different nicknames (or nickname slots if you want to look at it that
way) for an RBridge that has multiple nicknames. That way, for
example, if the number of distribution trees is constrained, you can
be sure they are spread across multiple RBridges rather than tending
to have all or nothing of the potentially trees rooted at each RBridge
with multiple nicknames.

Also, it would probably be a good idea to say that a nickname
configured with zero, the lowest priority, is saying that it does not
want to be a tree root and that it won't be one (except that if all
RBridge nicknames in a campus are configured with zero tree priority,
the highest priority based on tie-breakers is still a root so you will
always have at least one distribution tree).

Thanks,
Donald
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From: Radia Perlman <[email-masked]>
Date: Tue, 22 Sep 2009 22:27:13 -0700

I don't think this matters much either way. On the one hand, the only
real excuse for R1 having multiple nicknames is because someone thought R1
was well positioned, in terms of being central to the topology, or 
whatever, to
be a root, and want to be able to use multiple shortest-path trees from 
R1 as
the distribution trees.  So that would argue that a single priority would
be enough -- if one of R1's nicknames is not going to be one of the selected
tree roots, then R1 shouldn't be using that extra nickname. In other words,
if R1 is configured to take 6 nicknames, it's because the network 
administrator
wants R1 to be the root of 6 trees.

On the other hand, it's really just a matter of how one organizes the 
information
in the LSP, right? Whether the priority is associated with the nickname or
the RBridge? I don't think it complicates processing, or adds
a lot of overhead to an LSP. So I don't think it's harmful to make the 
priority be per nickname. It might
be nice to add words to the
spec to discourage people from thinking that lots of RBridges ought to 
get lots of nicknames.

And am I forgetting some other reason for an RBridge having multiple 
nicknames?

Radia

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From: Donald Eastlake <[email-masked]>
Date: Wed, 23 Sep 2009 11:30:58 -0400

Hi Radia,

On Wed, Sep 23, 2009 at 1:27 AM, Radia Perlman <[email-masked]> wrote:
> was well positioned, in terms of being central to the topology, or whatever,
> to
> be a root, and want to be able to use multiple shortest-path trees from R1
> as

That's a great argument assuming everything is going as the network
administrator intended. But lets say something goes slightly awry and
an RBridge gets attached or reset that has a low number of trees it
can calculate, say six. It seems better to make it possible for the
administrator to set things up so that those will be at six different
RBridges rather than having your hypothetical R1 being the root for
all 6 allowed trees.

> spec to discourage people from thinking that lots of RBridges ought to get
> lots of nicknames.

Yes, Bernard Aboba's remarks also encourage the addition of such words.

> And am I forgetting some other reason for an RBridge having multiple

I don't think so.

Thanks,
Donald

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From: Radia Perlman <[email-masked]>
Date: Wed, 23 Sep 2009 17:15:02 -0700

Donald,

yeah, I agree you should make the priority per nickname. There's no 
downside to
your proposed change, it's a really minor change, and I could imagine a 
case where there
are two well-situated root candidates; R1 and R2, and you want to have 
as many
trees as possible, half rooted at R1, and half rooted at R2, so that 
ingress guys near
R1 will choose to path split among the R1-rooted trees, and ingress guys 
near R2 will
choose to path-split among the R2-rooted trees.

So, yeah, by giving each of R1 and R2, say, 5 nicknames, with 
interleaving priorities,
you'll get half the trees rooted at R1 and half at R2.

Radia