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

[Rick Ernst]

Thanks everybody for all the feedback and information.  Between that and the
white paper I'm starting to feel comfortable with my decision-making
process. At worst, I can start asking more intelligent questions (and be
able to vet the answers) of Cisco.

One piece that I'm still unclear on is on CEF256/fabric cards and
connectivity to the rest of the system.  The white paper says:
  "- CEF256: The line card in this mode supports a connection into the
32-Gbps shared bus and the switch fabric-these line cards will use the
switch fabric for data switching when the Supervisor Engine 720 is
present-if a Supervisor Engine 32 is present it will revert back to using
the 32-Gbps shared bus."

The way that is written, a CEF256 card in a Sup720 equipped chassis will use
the 8Gbs fabric to move data around.  In a sparsely populated (eg 2 CEF256
cards) system there is more capacity on the shared 32Gbs bus than on the
fabric.  Am I misreading/misunderstanding?

Does forcing the card into flow-through mode address this?

Rick


On Fri, Nov 6, 2009 at 5:55 AM, Geoffrey Pendery <geoff at pendery.net> wrote:

>
> If you have a Sup with no fabric (like Sup 1A, or Sup 2 w/o SFM, or
> Sup 32) the switch will run in "Flow-Through" mode, meaning that each
> time a packet is received, the entire packet is sent on the shared
> bus, so it's seen by the Sup and all line cards.  This will only get
> you up to 15 Mpps, and a theoretical max of 32 Gbps (likely lower in
> practice).
>
> If you have a fabric Sup and fabric line cards, but at least one
> Classic line card, the switch will drop into "Truncated" mode.  This
> is likely what someone was referring to when they told you "lowest
> common denominator".  The classic cards will still send the whole
> packet, like in flow-through mode, but the fabric cards will send only
> the headers, and send the data portion to the Sup via fabric.  This is
> still limited to 15 Mpps, but because the data flows via fabric, you
> can squeeze some extra bandwidth out.
>
> Lastly, if you have no Classic cards present in the chassis, it can go
> into Compact mode, where only compressed headers are sent via the bus,
> all data flows via fabric.  This gets you up to 30 Mpps and your
> theoretical 720 Gbps of total forwarding capacity.
>
>
>