Subject: [rbridge] FW: Time to summarize "forward or block" BPDU thread

From: kreeger at cisco.com (Larry Kreeger (kreeger))
Date: Wed, 19 Oct 2005 15:25:37 -0700

Radia,

I think I now understand now that the DR election happens only between
RBridges on a single shared link - and does not require usage of any
RBridge topology information.  It would help clarify the data flow for
me (and hopefully others) if, given the network diagram below, you could
please give a step by step description of how a broadcast frame sent by
host A gets through each Bridge and RBridge in this network in order to
be received by hosts B,C,D,E if we assume that RB1 is the Designated
RBridge on the "link" created by the Bridged network formed by Bridges
B1,B2,B3,B4.

  B    C    D    E
  |    |    |    |
 RB1  RB2  RB3  RB4
  |    |    |    |
  B1---B2---B3---B4
                 |
                 A

Thanks, Larry

-----Original Message-----
From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
Behalf Of Radia Perlman
Sent: Wednesday, October 19, 2005 9:57 AM
To: Developing a hybrid router/bridge.
Subject: Re: [rbridge] FW: Time to summarize "forward or block" BPDU
thread

The IS-IS Hello protocol is for electing a single unique switch per
link. It doesn't matter which one it is.

On the other hand, the spanning tree protocol is picking a special
single unique bridge per link...the one that is closest to the root.

The spanning tree algorithm is choosing a tree of shortest paths from
the root.

IS-IS is not trying to do that. It is just electing one guy on a link to
do special link-specific things.

Radia

---

From: Radia.Perlman at Sun.COM (Radia Perlman)
Date: Wed, 19 Oct 2005 15:56:45 -0700

Step 1: The bridges create a single broadcast domain, as seen by the
RBridges. So the RBridges see the following picture:

  B    C    D    E
  |    |    |    |
 RB1  RB2  RB3  RB4
  |    |    |    |
 ---------------------
                 |
                 A

Step 2: RB1 is elected DR, so it is the only one that will
encapsulate or decapsulate to/from that link.

Step 3: A transmits a broadcast or multicast packet

Step 4: RB1 receives it (RB2, RB3, and RB4 all throw it
away since they are not allowed to process native (non-encapsulated)
packets

Step 5: RB1 encapsulates it to be sent over spanning tree
"ingress RB1"

Step 6: RB1 forwards the encapsulated packet onto the link,
with an outer header with a multicast address "all RBridges"

Step 7: RB2, RB3, and RB4 all receive the packet, and decapsulate
it in order to send to their upward link (to C, D, and E,
respectively).

Step 8: (which could certainly happen after 5 or 6...doesn't have
to be in order). RB1 decapsulates the packet and sends it to B.

************
Hope that helps...

Radia

Larry Kreeger (kreeger) wrote On 10/19/05 15:25,:
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