Subject: [rbridge] seeking inputon abstractfor problemand applicabilitystatement

From: petera at nortel.com (Peter Ashwood-Smith)
Date: Fri, 14 Oct 2005 14:18:15 -0400

>> I think I missed something, what breaks if we block BPDUs ?

>If nothing, then lets. For bridges too.
> Now what happens? (i.e., if bridges block BPDUs) - won't that kill the
spanning tree?

 Yes of course if you block a BPDU the bridge will stop the STP tree at
that point. Thats exactly what we are trying to do. Stop the STP
computed portion of the FULL tree at that point. 
  
 With BLOCK. Each protocol computes its own smaller part of the bigger
spanning tree.
 STP computes using BPDUS etc. and Rbridge computes using
IS-IS/Krusal's.

 When done you have a bunch of unconnected spanning trees which are made
into one big tree by the designated rbridge concept i.e. single link.

 To convince yourself that this really works. Draw an arbitrary tree on
a piece of paper. Then draw another disjoint tree somewhere else on the
paper. You can make one big tree by connecting any two of the points
between the two trees and you cannot create a loop with single link.
That is why BLOCK works.

 Peter

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 14 Oct 2005 13:54:55 -0700

>  STP computes using BPDUS etc. and Rbridge computes using

Eve if each edge is its own tree, it can touch the rbridge in more than
one place (if not, why not?)

It seems like the internal spanning tree needs to 'pick' one of those
tree attachment points to join to, so that one overall tree is picked.
But that's back to PARTICIPATE rather than BLOCK.

> into one big tree by the designated rbridge concept i.e. single link.

The rbridge isn't acting like a link. It acts like a link in TRANSPARENT
mode; in that case, when one edge tree send a BPDU to the link, everyone
on the link (all the other edge trees) will receive it.

> between the two trees and you cannot create a loop with single link.

The only way it'll work like a single link is in TRANSPARENT - consider
a spanning tree that touches the 'connecting link' (the rbridge) in more
than one place (which is valid). The reason the spanning tree would pick
only one of those to join is because of the BDPU messages.

Joe
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