Subject: Re: [rbridge] Mail regarding draft-ietf-trill-rbridge-protocol

From: Donald Eastlake <[email-masked]>
Date: Wed, 4 Aug 2010 12:52:58 -0400

Hi Maria,

On Wed, Aug 4, 2010 at 8:26 AM, <[email-masked]> wrote:
>
> Good morning,
>
> Im reading the RFC for TRILL and its benefits for multipathing and loop
> avoidance. I have some doubts, among them i would like you can explain me
>
 1. if there is just one DRB per campus or can be more,
 The DRB (Designated RBridge) in TRILL is the same as the "Designated
Router" or "Designated Intermediate System" in IS-IS. There is one per link,
not one per campus, using the word "link" as defined in the protocol draft.
You may find it helpful to look at the IS-IS routing protocol.

> 2. Also for a network similar to the scheme attached in this mail:
> <>
> This is a network example im trying to understand in order to ease my
> comprehension of the protocol. Consider this four Rbridges interconnected
> RB1, RB2, RB3 and  App Fw Vlan x. Imagine they are connected through normal
> Bridges (therefore frames must be with an outer header ehternet)
>
OK.  Keep in mind that Appointed Forwarder status is a characteristic of an
RBridge port, not of the entire RBridge.

> In case Host B wants to send a packet to Host A, if these two belong to
> vlan x, the native packet will go to the spanning tree formed by Bridge 4, 6
> and 5 up to RB1 wich could be the root tree (A.3.3
> draft-ietf-trill-rbridge-protocol-16) ; then RB1 will encapsulate in TRILL
> Format and send the packet to the Appointed Forwarder for VLANx who will
> know to which path send this packet towards Host A. ?
>
(A.3.3 is about an optional feature and is not too relevant to normal
RBridge operation.)

Well, first I'll talk about this after MAC addresses have been learned:
   The port on RB1 that is connected to Bridge 4 is the only RBridge port
connected to that bridge LAN so it must be acting as the appointed forwarder
for VLAN X, otherwise the frame would be blocked at that port.
   Since I am assuming that MAC addresses have already been learned, RB1
will know that the MAC address and VLAN for Host A is attached to the RB you
have labeled "App Fwd" which I will call RBAF. So RB1 encapsulates the frame
putting the nickname for RBAF in the Egress Nickname field. In this case the
shortest path is just one hop, but in the general case their could be
intermediate transit RBridges that keep forwarding the frame, hop-by-hop, to
RBAF.
   When the frame arrives at RBAF, it recognizes itself as the Egress,
decapsulates the inner frame, looks up the MAC address and VLAN to find out
which port to output it on. Since, in this case, there is only one RBridge
port connected to the bridged LAN consisting of Bridge 2, Bridge 3, and
"Root Bridge", that port has to be the Appointed Forwarder for any VLANs
that are going to get service from the RBridge campus for things attached to
those bridges.
   The bridges then direct the frame down to Host A.

If MAC addresses had not yet been learned, when the frame arrived at RB1, it
would have been encapsulated as a multi-destination frame, sent on a
distribution tree, and output on all ports in the campus which are Appointed
Forwarders for the frame's VLAN.

Note that there are two separate spanning trees in your diagram. One
composed of Bridges 4, 5, and 6 and one composed of Bridges 2, 3, and the
one you label "Root Bridge". RBridges terminate the spanning tree, just like
Layer 3 routers do.

> Please I would appreciate your help with my doubts and thank you for your
> attention.
>
I hope the above is helpful,

Thanks,
Donald

> *Belén Egüez*
> FT/RD/RD/BIZZ/CMC/LSD
> Stagiaire de R&D - Services LAN et Stockage de Données en réseau
> tél. 01 45 29 62 42
> [email-masked]* <[email-masked]>
>
>
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