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

[Radia Perlman]

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,:
> 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
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