Subject: [rbridge] PS issue #11

From: Eric.Gray at marconi.com (Gray, Eric)
Date: Thu, 8 Dec 2005 14:27:03 -0500

Spencer/Joe,

	This E-Mail is rich with stuff I think needs clarification.

	The statement that all bridges are "outside" is part of the
confusion generating definition of "external" included in the TRILL
architecture draft.  Joe has indicated that he needs to change his
referencs to "external protocol interactions" to "interactions with
802.1D bridges".  In general, we need to move away from thinking of
interactions as "internal" or "external" based on who you're talking
to rather than where they are.

	There is more to the notion of "inside" (can we use consistent
terminology?) - or "internal" than the fact that traffic is tunneled
between RBridges.  There is also the fact that RBridges discover one
another over internal links, that IS-IS is used to determine routing
over internal paths and that - because the traffic is tunneled within
an RBridge campus - the traffic arriving at any ingress may take more 
than one path to arrive at a given egress, as long as appropriate 
steps are taken to ensure that frame ordering for individual frame 
streams is usually preserved.

	Also, unless things have changed in how bridges typically handle
STP, the opportunity for race conditions is very small.  Forwarding
between RBridges will not occur until STP has been resolved among any
bridges involved in the connecting links.  Bridges usually (I can't 
recall if it is a requirement) do not forward while resolving STP.  I
am fairly sure that this is also true for RSTP.

	There will be SPF recalculations, however, if the RBrdiges do
not employ a hold-down timer sufficient to allow for all links to
become available.

--
Eric

--> -----Original Message-----
--> From: rbridge-bounces at postel.org 
--> [mailto:rbridge-bounces at postel.org] On Behalf Of Spencer Dawkins
--> Sent: Wednesday, December 07, 2005 4:23 PM
--> To: Developing a hybrid router/bridge.
--> Subject: Re: [rbridge] PS issue #11
--> 
--> Hi, Joe,
--> 
--> Thanks for your help as I catch up ...
--> 
--> >> In the following picture, are the (inside?) bridges in the middle of
the
--> >> rbridge campus part of the same (outside?) spanning tree, or does
what
--> >> happens inside the campus show up outside the campus?
--> >
--> > The same issue applies - all bridges are 'outside'. It's only traffic
--> > that's considered to be inside or outside. This is because the traffic
--> > between rbridge nodes is tunneled, and it is the tunnel that is
--> > 'inside'. As a result, bridges may transit tunneled traffic, but they
--> > never appear as bridges inside the tunnel, and so are never 'inside'
the
--> > rbridge.
--> 
--> I'm wondering if we have the potential for race conditions as bridges 
--> between rbridges recompute a spanning tree while rbridges are
distributing 
--> link state updates, but at worst, I'm thinking this might involve one
extra 
--> path change during routing changes - not oscillating. Maybe this is
worth 
--> revisiting when we're more solid on the protocol description, but it's 
--> probably premature now.
--> 
--> Again, thanks!
--> 
--> Spencer 
--> 
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