[c-nsp] 6500 SUP720 High Latency and Jitter issues

[Simon Leinen]

Jared Mauch writes:
> On Wed, May 25, 2005 at 11:26:54PM +0200, Gert Doering wrote:
> [...] this is part of knowing your platform.  The '76k' (i've
> renamed it since it's confusing to have the 7600/6500 name, i
> encourage everyone to call it by this name :) is a software based
> platform, with 'mls' hardware assist for some features.

I don't think that this is a useful way to look at it.  Besides the
general confusion of the "software vs. hardware" terminology, I find
it more useful to think of the Catalyst 6500/7600 OSR (76k? Shouldn't
you call it 7k6?) as a "hardware-based" (ASIC/TCAM) platform, which
has a general purpose CPU that can be used to switch packets "in
software".  But software switching should remain the absolute
exception.

> If you add a distributed linecard, they will be handled in the
> linecard (in most cases) that can be seen by doing a 'sh cef line'
> to know what slots are likely to be hardware switched.

> 	This is similar to the 7500, where if you had a VIP linecard,
> packets would (if configured for dcef) be handled in a distributed
> fashion, but with older linecards, it would be handled on the RP.

Yes, DFCs are similar to VIP in the sense that you can use them to
distribute the forwarding work.

But a DFC always offloads forwarding work from the PFC, and a VIP
offloads forwarding work from the main (RSP) CPU.  And a PFC/DFC can
switch 20-30 Mpps, while a VIP or an RSP - and the general-purpose CPU
on the MSFC - can handle something like 100-200 kpps(?).

So DFCs are nice if you are starting to reach the 30 Mpps limit of the
PFC (our OSRs all switch <500 kpps, so we don't have a single DFC),
but they don't help when your MSFC is loaded.  On the other hand, the
MSFC shouldn't do anything other than run BGP, OSPF/IS-IS, SNMP and
the CLI.

So for you the 76k is a distributed software-based platform that can
use "hardware" to speed up some frequently-used features (such as IP :-).

For me the 7K6 is a centralized hardware-based platform that has some
software so that I can log into it and play with it, and that can be
extended with distributed hardware forwarding ("decentralized") if
necessary.
-- 
Simon.