On Mon, Jun 15, 2020 at 10:37 AM <[email protected]> wrote: > > Hi Robert, > > > > > It’s very clear that this is inadequate. > > > > Doesn't this really depend how you architect multiple levels ? Sure you > have some physical topology - but it seems to me that the trick is in > properly laying out levels on top of them and between them. > > > The very pragmatic viewpoint is that network architects will construct the > physical topology that best suits their traffic pattern and that trying to > contort that physical topology into some kind of hierarchy will create > additional (and perhaps significant) cost. People don’t want to do that. > They want scalable networks that match their physical topology. For this > to work, our hierarchical abstractions need to be independent of the > topology and that forces us into having transit areas. > > > PNNI had transit areas in hierarchy working but the trick was connection setup cranck-back. Such a thing would work for RSVP or any of the stateful connection setups but alas, this is not fashionable right now. Unfortunately, generic hierarchy with reachability summarization ends up with very sub-optimal routing or black-holing on aggregates since we cannot "back-off" generic LPM packet forwarding when we realize we're @ a dead end due to aggregation. To prevent bi-furcation of topology or transit horizontals several solutions exist, one of which (configure hierarchy statically everywhere) the current draft has in now but alas, the topology is star of stars (you can actually see CLOS conceptually as something like this as well ;-)
my meta 2c -- tony
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