Subject: [rbridge] Margaret's question about VLANs

From: Radia.Perlman at Sun.COM (Radia Perlman)
Date: Mon, 02 May 2005 18:03:46 -0700

Margaret Wasserman wrote:

>(2) How will TRILL be compatible with existing (dynamic and 
>non-dynamic) VLANs?
>

I was thinking about why it might be relevant how an RBridge learned 
that its link
was in VLAN A, whether it was configured, or found out because an endnode
attached (and the endnode knows what VLAN it's in or the RBridge figures it
out based on the endnode's IP address or something?) The current 
proposed design
seems compatible with both dynamic and non-dynamic VLANs.

However, now that you mention it, assuming by "dynamic VLAN", you mean
a link that becomes a VLAN A link because a VLAN A node attaches
to it, and stops being a VLAN A link when the endnode goes away),
I think there is one optimization I'd propose to make
dynamic VLANs work better, and which probably we should do anyway, even 
without
dynamic VLANs, which is to allow configuration of "priority" for an 
RBridge to
be chosen Root for a spanning tree to be computed from the link state 
database.
We should have an RBridge R not only inject "I am attached to VLAN A" into
the link state protocol, but it should also have a configurable 
priority, so that
it says "I am attached to VLAN A, and I have priority x for being the 
tree root for
the VLAN A spanning tree". That way, if R keeps "changing its mind" 
about whether
it's attached to VLAN A (based on whether there are any endnodes 
attached at this
moment), it will be less disruptive to the VLAN A spanning tree (since the
root won't be changing). Even without VLANs, where the entire campus is one
broadcast domain, I could imagine someone wanting to tweak which Rbridge is
the root of the computed spanning tree (the one for unknown 
destinations, and for
multicasts).

Even if it's not a parameter, I could imagine having an RBridge inject a 
default priority
if it's hard-configured to be attached to VLAN A, and a different 
default priority if
its attachment to VLAN A might be transitory (because it's based on what 
endnodes happen
to be attached).

Radia

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From: Radia.Perlman at Sun.COM (Radia Perlman)
Date: Mon, 02 May 2005 18:06:21 -0700

Radia

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From: greg.daley at eng.monash.edu.au (Greg Daley)
Date: Tue, 03 May 2005 12:14:56 +1000

Hi Radia,

Radia Perlman wrote:
[cut]
> the root of the computed spanning tree (the one for unknown 

This actually helps to reduce the total cost of bridging in the spanning
tree where there is assymmetric traffic in the bridged network.

The existing STP doesn't include measurements of traffic flows in its
root bridge calculations.

If there's an RBridge which is likely to be the STP source or sink of a
lot of traffic, I think it's easy to show that even with symmetric
segment cost metrics, the sum of the traffic costs to the exit point is
lower when the entry/exit point is root bridge.

I'd have to revisit the calculation of this, but the idea has been
kicking around for a while. I'll post some of my thinking on this in the 
next mail.

> its attachment to VLAN A might be transitory (because it's based on what 

This is a fair assumption.  I'm not sure how IS-IS would do this, but
if it was OSPF, you could have the Designated Router with the lowest
metric (likely to be selected as the best), and the Backup Designated
Router as the next most likely (with a slightly bigger preference
value), for simplicity's sake.

When links went up or down, and an STP recalculation was required, the
routers could advertise their preferences based on their Routing
protocol status within the LAN (and maybe use their 802 MAC addresses
as a tiebreaker).

Greg