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

[Gert Doering]

Hi,

On Thu, Nov 05, 2009 at 11:11:56PM -0800, Rick Ernst wrote:
> What I think the document says is:
[..]

As far as I understand the architecture, all of this is correct :-)

>   - Mixed 8/20Gbs line cards can be used. 20Gbs is not lost if 8Gbs is
> present.
> 
> I'm most confused on the 8Gbs limit and how it relates to the Supervisor and
> line cards.  

65xx cards (like the WS-X6516) have an 8Gbps fabric connection, 67xx cards
(WS-X6724-SFP) have 20Gbps fabric connection.

Sup2+SFM has 8Gbps fabric.

Sup720 has 20Gbps fabric that can also run at 8Gbps - and *as far as I
understand* - this is independent among line cards, so you can have one
WS-X6516 running at 8Gbps and one WS-X6724-SFP running at 20Gbps.

> Other discussions I've had indicate that some combination of
> line cards can bring the whole system down to the lowest common
> denominator.  

There's two sides to "lowest common denominator" - bus/fabric (so if you
have a Sup2 without fabric module, only shared bus for you...) - and 
PFC revision.  There's Sup720/3A, /3B and /3C, and all of these can come
with "-XL".

So - if you have a Sup720/3C-XL with 1 million TCAM entries and 96k MAC
table entries, and add a line card that has an DFC-3A on it, the whole
system will fall down to "3A, no XL" level -> no MPLS, 256k TCAM entries,
32k MAC table entries.

This is only relevant if you have DFCs in the system.

> Am I on track? Where does oversubscription on line cards come
> in? Is there something else I haven't covered?

Oversubscription is the next independent gotcha - for example, the 
6724 card has 24 gbit ports, but only 20 gbps fabric connection
(which is not that bad, given that in practice, nobody runs all 24 ports
at 100% line rate all the time).

The 6708 10G card has 8x10 gbit externally, but only 40 gbps fabric
connection - but it has a DFC and can do local switching without
going to the fabric, so depending on your traffic pattern, it's more
or less oversubscribed...

The 6704 10G card has 40 gbps fabric connection, but a somewhat slow 
internal ASIC, so it won't do more than 35 Gbit/s (or so) in total...

So you also need to take into account the specifics of the line card
you're planning to use.

> 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.

The architecture *is* a bit confusing :-)

gert
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USENET is *not* the non-clickable part of WWW!
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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