[NTLUG:Discuss] Ethernet Switches

[Dan Carlson]

I don't think there is anything inherent in switching that precludes
supporting broadcast.  I think whether it is supported and how well it
performs would depend on the implementation.

I recently worked on a project that used Broadcom BCM5318 switch devices.
They support both broadcast and multicast at full wire speed.  For a switch
such as the BCM5318 that supports both managed and unmanaged modes the
behavior can differ between the two modes.  In the managed mode you have
direct control over how addressing is handled, and you can set it up so
that it works and works efficiently, or not.  In the unmanaged mode it all
works by default (if properly implemented), but maybe not as efficiently as
if you created a custom configuration specific to your needs in the managed
mode.

Most of the low-end switches you can buy in retail stores operate in an
unmanaged mode, and they should support broadcast by default, but it may be
the case that they don't all support it at full wire speed to all ports
simultaneously.  If there are any that don't support broadcast, I wouldn't
expect them to be very successful in the market.

I can think of three ways to determine if a given switch supports broadcast
and if it supports it efficiently:  1) Find a support engineer that works
for the manufacturer that will give you a straight and believable answer,
or 2) Take the switch apart, determine which switch device it uses, and
check the specs for that switch device, or 3) Actually try it and do your
own benchmarking and performance measurement.

One advantage of switches over hubs is that you can string more of them
together and still stay within ethernet cabling specifications, which may
help the the creation of a 200 port network, depending on the physical
layout.

If I interpret 802.3-2000 correctly, a switch and a bridge are logically
the same thing.  It appears that "switch" is the newer term, and that they
typically come in smaller packages and cost less.

Dan Carlson

----- Original Message -----
From: "Steve Baker" <sjbaker1 at airmail.net>
To: <discuss at ntlug.org>
Sent: Tuesday, October 09, 2001 12:07 AM
Subject: Re: [NTLUG:Discuss] Ethernet Switches


>
> On a similar topic (switches versus hubs)...
>
> One of the applications I've been working on uses exclusively broadcast
> mode UDP messages - presumably in that context, I wouldn't benefit from
> paying the extra and getting a switch instead of a cheap hub.  I
recollect
> hearing somewhere that some switches don't even support broadcast at
all -
> or do it very slowly by sending the message consecutively to all the
outputs
> instead of to all of them in parallel.
>
> Does anyone know if that's true?  And if so, whether there is a way to
tell
> which switches support broadcast 'properly' and which either do it
nastily
> or not at all?
>
> Right now, I'm only using a handful of machines - but what if I wanted to
> connect up (say) 200 machines and use broadcast to all of them in
parallel?
> I'm betting there aren't too many 200 port *hubs* out there - can I
cascade
> them?  Is this where a 'bridge' would be appropriate?
>
> Low latency and high bandwidth are important here - I'll be looking for a
way
> to do this with 1GHz Ethernet too.
>
> (This is a *really* specialised Beowulf-like Linux application)
>
> ----------------------------- Steve Baker -------------------------------
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