[c-nsp] CEF and multicast on a 3750

[Gert Doering]

Hi,

On Tue, Jun 26, 2012 at 09:07:32AM -0600, John Neiberger wrote:
> These are extremely expensive real-time video encoders taking in
> baseband video and outputting multicast MPEG at a constant bit rate
> for customer-facing video feeds. Any pauses would be unacceptable, as
> are dropped packets, unfortunately for this site.  hehe

Bit rate is never "constant".  It's always "constant if averaged over
a certain time window".

If you send 20 packets per second, and send them in bursts of 20 packets
and then 0.99s of silence, you are *still* sending a "constant bit rate
of 20 packets per second", if averaged over a time window of 5 seconds.

Unless you are really sure (by having done a wireshark / tcpdump dump
with time stamps directly on the encoder port, with no other gear in 
between) that packets are space out evenly, with only one packet ever
sent per burst, and then a delay that's the same for every packet, do
not assume that "constant bit rate" is really non-bursty at the microsecond
level.

We've learned this the hard way with our audio streaming setup - some
streaming server software is quite good at pacing the packets (windows
media server), some is particularily bursty (icecast, if I remember
right).  Of course I do not know the application that you're using, just
trying to break some "it can not be this!" assumptions.

> I'm engaging our Cisco team on this to have them investigate queueing
> and buffer issues. I don't think they've looked at that yet.

"We do not have buffer issues in these switches!  They do not HAVE 
enough buffers to have issues with them!"

gert

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