Subject: [rbridge] query regd. TRILL scenario

From: Joydeb Roy <[email-masked]>
Date: Mon, 2 Jan 2012 15:36:14 +0530

Hi Authors,

Please let me know if my understanding is proper in the below case. If yes,
then please clarify the doubts and if no, then please correct wherever
applicable.

Generally, RBridges learn about end stations on access ports and they
communicate with each other to form adjacency through trunk ports.  But if
we consider the following diagram –

                                  +---------+
+---------+                       |   RB1   |
|   End   |        +-----+ ------ +---------+
|         | ------ | Hub |
| Station |        +-----+ ------ +---------+
+---------+                       |   RB2   |
                                  +---------+

How do the involved ports in RB1 and RB2 behave in this case? Do both of
them manage to learn about the End Station and one of them becomes the DRB
based upon the configured priorities? Also, do they manage to form an
adjacency between them? If so, then how do they manage so in spite of the
ports being designated as access?

Thank you.

Best regards

Joy
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From: Donald Eastlake <[email-masked]>
Date: Mon, 2 Jan 2012 09:21:24 -0500

Hi Joybed,

> we consider the following diagram –

TRILL tries to be zero/minimum configuration. So by default, RBridge
ports are universal ports that can handle both native and TRILL Data
frames. RBridge access and trunk ports are ports that have been
configured to restrict them. While this is commonly a good idea, it is
not necessary for correct operation.

In any case, TRILL Hellos are always sent out RBridge ports unless the
port is disabled or configured as point-to-point (in which case
point-to-point Hellos get sent on them).

As above, TRILL Hellos are still sent out the ports of RB1 & RB2
whether or not the "access port" configuration bit has been turned on
for those ports. So they form an adjacency and one is elected DRB on
the link. By default, the DRB will handle all the native traffic
to/from the End Station. The other RBridge will see all that traffic
but will throw it away because by default it is not the Appointed
Forwarder for any VLAN on the link. However, the DRB can assign some
or all VLANs to the non-DRB RBridge as Appointed Forwarder.

> Joy

Hope the above helps.

Thanks,
Donald
=============================
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 155 Beaver Street, Milford, MA 01757 USA
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From: Joydeb Roy <[email-masked]>
Date: Tue, 3 Jan 2012 12:21:57 +0530

Hi Donald,

Thanks a lot for your reply. The understanding is indeed much better now.
Just another query though regarding the AVF part you mentioned in your
comments -

Now, I understand that the default the DRB will carry all the traffic to
and fro the end-user. But when the DRB does decide to distribute the
traffic to other rbridges then what propels it to make that decision?

The last sentence of the Section 2.2 from the RFC 6439 states -

"How the DRB decides what other RBridges on the link, if any, to appoint
forwarder for which VLANs is beyond the scope of this document."

So, is there any other standard document that I can refer to? Is the
behavior totally dynamic in nature e.g. the DRB takes such decision through
the usage of some algorithm? Or should it be configurable, so that the
Network Admin takes the last call?

Thank you.

Best regards
Joy

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From: Donald Eastlake <[email-masked]>
Date: Fri, 6 Jan 2012 22:05:56 -0500

Hi Joydeb,

On Tue, Jan 3, 2012 at 1:51 AM, Joydeb Roy <[email-masked]> wrote:
> Now, I understand that the default the DRB will carry all the traffic to and
> fro the end-user. But when the DRB does decide to distribute the traffic to
> other rbridges then what propels it to make that decision?
>

I kind of think of how the DRB decides what forwarders to appoint as
an area where implementors can display their individual brilliance :-)

Seriously, there are a lot of factors that could be used in such
decisions. I don't see how it could be standardized. But you might
look at draft-zhang-trill-vlan-assign-02.txt and at Section 10.2 of
draft-eastlake-trill-rbridge-clear-correct-02.txt.

By the way, in connection with your previous question, you might look
at Section 6 of draft-eastlake-trill-rbridge-clear-correct-02.txt.

Thanks,
Donald