Subject: [trill] MD-levels and TRILL

From: Anoop Ghanwani <[email-masked]>
Date: Tue, 6 Nov 2012 15:09:50 -0800

There are a couple of challenges that I see with trying to
define the use of MD-levels with TRILL.

MD-levels in the IEEE world were used to creates multiple
levels at which CFM can be run.  For example, if I buy a service from a
provider and start running CFM, only my boxes (my network and
the edges of the provider network) will see and respond to those
CFM messages.  Maybe this type of service model doesn't
make sense for TRILL because, if the provider implements
the transport using RBridges, then the customer and provider
will have to make sure they assign their nicknames from the
same space.  But let's leave that issue aside for now...

The first challenge is that TRILL depends on the Hop Count
for some of the OAM messages to decide whether they are
processed by an RBridge.  This means that it is not possible
to send these messages in a way that is transparent to
set of RBridges.  So pretty much all OAM messages must
be processed by all RBridges along a path regardless of
level.  So I'm not sure what the concept of MD-level would
buy us.  At best it may be used for filtering and that too in
the control plane, but I personally don't see much value there.

The second challenge is related to accessing the MD-level
withing the frame to know whether or not to process it.
Because the MD-level would follow the 128-byte entropy,
I am not sure that existing silicon, and maybe even emerging
silicon, will be able to access those bits, so that level of
filtering may have to be done in software.  I think this should
be clearly called out in the draft.

Anoop

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From: Donald Eastlake <[email-masked]>
Date: Sat, 10 Nov 2012 21:07:06 -0500

Hi Anoop,

> define the use of MD-levels with TRILL.

I agree that there is still design work to be done.

> same space.  But let's leave that issue aside for now...

Generally I would expect provider facilities to be transparent to
customer TRILL OAM.

I can see reasons for 3, possibly 4, different levels within the
customer TRILL level. First of all, there is multi-level IS-IS which
would yield two levels, traditionally called level 1 and level 2. Then
there are ideas like that in draft-wu-trill-lsp-ext-tree-distr-opt
which could produce sub-areas within a level 1 area which we might
call level 0. Then there are idea like that in
draft-ietf-l2vpn-trill-evpn which could lead to a level above level 2,
which you could call level 3. Of course, if you were using the
MD-leels, you could map these into different numbers in the range 0 to
7.

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 155 Beaver Street, Milford, MA 01757 USA
 [email-masked]

> within the frame to know whether or not to process it.