[c-nsp] Please help clarify bus/fabric terminology on the 6500/7600

[Rick Ernst]

I'm trying to wrap my brain around Cisco's document on the 6500/7600
technology:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html

Terminology on the bus architecture and switch fabric are becoming less
confusing to me the more I read it, but I'm still not comfortable with my
level of understanding.
What I think the document says is:

  - The 32Gbs shared bus is the path between the supervisor and individual
line cards. Line cards do not move data between each other; traffic must
pass through the Sup.
  - The raw capacity of the 32Gbs bus is just that; 32Gbs across the entire
bus, combined across all cards
  - The switch fabric is single or dual channel 20Gbs, dual channel just
allowing higher port/speed-density on the line cards
  - The 20Gbs fabric is used to transfer traffic directly between
DFC-enabled line cards, bypassing the Sup.
  - The 20Gbs fabric is not shared, each DFC line card can talk to any other
DFC line card at 20Gbs up to a potential aggregate of 720Gbs
  - CEF and dCEF simply refer to whether the line card has a DFC
  - CEF256 using 8Gbs of the fabric, CEF720 uses 20Gbs
  - "Classic" line cards use only the 32Gbs bus.
  - Usage of 8Gbs or 20Gbs on the fabric is dependent on the line card and
the Sup.
  - Sup720 allows 20Gbs, others are only 8Gbs
  - Mixed 8/20Gbs line cards can be used. 20Gbs is not lost if 8Gbs is
present.
  - The full "720Gbs" capacity is all dual-fabric line cards with DFCs

I'm most confused on the 8Gbs limit and how it relates to the Supervisor and
line cards.  Other discussions I've had indicate that some combination of
line cards can bring the whole system down to the lowest common
denominator.  Am I on track? Where does oversubscription on line cards come
in? Is there something else I haven't covered?

Sorry for the laundry list.  I'd rather make sure I'm clear in my head
before the design, then find a gotcha after it is too late.

Thanks!