Broad on a switch?

Hi there,

I know this is a little off topic but I am not sure where I shoud post
this question. Any hint where I should post is welcome.

The problem is we are doing parallel scientic computation on a cluster
we build on our own which is running linux. All the nodes are connected
to a Gigabit switch. The work is both CPU and communication intensive.

The question is how is broadcast actually done when the masternode tries
to broadcast data to all the other nodes? I am not a computer scientist
but I kind of know that on a hub any message is broadcast on the same
network and everybody could get it. While for my understanding, for
switch, there is no way do such kind of broadcasting. So although in
most cases, a switch is suprior to a hub but if what I need is to
broadcast large amout of data quickly through the local network, maybe
switch is not as good as a hub. Is that right?

If this is true, another question arises: how can we improve the
broadcasting on a switch? I searched on goole and found somebody
mentioned VLAN but have no idea of what it is. Could you give me any advice?

Thanks.

Again, this might have nothing to do with linux, although we are running
linux for this case. I apologize.
--
Regards
Shi Jin
(E-Mail Removed)
http://jinshi.dhs.org

Shi Jin <(E-Mail Removed)> wrote:
> Hi there,
>
> I know this is a little off topic but I am not sure where I shoud post
> this question. Any hint where I should post is welcome.

Find a networking newsgroup, not a linux newsgroup.

> The problem is we are doing parallel scientic computation on a cluster
> we build on our own which is running linux. All the nodes are connected
> to a Gigabit switch. The work is both CPU and communication intensive.
>
> The question is how is broadcast actually done when the masternode tries
> to broadcast data to all the other nodes? I am not a computer scientist
> but I kind of know that on a hub any message is broadcast on the same
> network and everybody could get it. While for my understanding, for
> switch, there is no way do such kind of broadcasting. So although in

Nope. A switch will filter packets to a known dest ether address such that
only the target gets the packet. For broadcast, or any dest addresses that
the switch doesn't know about, the packet gets flooded just like in a hub.

> If this is true, another question arises: how can we improve the
> broadcasting on a switch? I searched on goole and found somebody
> mentioned VLAN but have no idea of what it is. Could you give me any advice?

Yup. Find yourself a network engineer. (VLANs, BTW, have nothing to do with
what you're asking about)

--
Frank Sweetser fs at wpi.edu
WPI Network Engineer

"Shi Jin" <(E-Mail Removed)> wrote in message
news:bm45rm$8uo$(E-Mail Removed)...
> Hi there,
>
> I know this is a little off topic but I am not sure where I shoud post
> this question. Any hint where I should post is welcome.
>
> The problem is we are doing parallel scientic computation on a cluster
> we build on our own which is running linux. All the nodes are connected
> to a Gigabit switch. The work is both CPU and communication intensive.
>
> The question is how is broadcast actually done when the masternode tries
> to broadcast data to all the other nodes? I am not a computer scientist
> but I kind of know that on a hub any message is broadcast on the same
> network and everybody could get it. While for my understanding, for
> switch, there is no way do such kind of broadcasting.


From my understanding, there is such a mode. the ethernet packet has headers
set that make it a broadcast packet,
and the switch broadcasts it.

The switch doesnt broadcast packets that are not set to be broadcast
packets.

Simple.

On Fri, 10 Oct 2003 13:34:41 +1000, Leon The Peon wrote:

>
> "Shi Jin" <(E-Mail Removed)> wrote in message


>> The question is how is broadcast actually done when the masternode tries
>> to broadcast data to all the other nodes? I am not a computer scientist
>> but I kind of know that on a hub any message is broadcast on the same
>> network and everybody could get it. While for my understanding, for
>> switch, there is no way do such kind of broadcasting.

It might actually broadcast the data (i.e. use a destination address of
ff:ff:ff:ff:ff:ff in the ethernet frame). This is a very efficient way of
distributing common information if ALL HOSTS on the segment need to hear
it.

Alternatively it might use multicast, a special mode of broadcast which is
targeted at a specific group of hosts. This is only efficient if the
switch infrastructure understands multicast and stops it from reaching
hosts that don't want it.

Finally it might use plain old multiple unicasts, one for each recipient
host. This is the least efficient because the master node needs to send
the same information to every host individually, and maintain the TCP/UDP
connection for each one.


> From my understanding, there is such a mode. the ethernet packet has headers
> set that make it a broadcast packet,
> and the switch broadcasts it.
>
> The switch doesnt broadcast packets that are not set to be broadcast
> packets.

Except of course for:
1) Multicast packets
2) Packets for unknown MAC addresses

(I'm not going to get all pedantic and point out that switches forward
frames, not packets, because someone would undoubtedly be forced to point
out that a "Layer 3" switch does, and then we'd have to enter into the
discussion of how L3 switching is actually implemented by various
manufacturers...)


--
David Haggett
Linux user since 01/01/2003
Email: david<at>haggett<dot>demon<dot>co<dot>uk

On Thu, 09 Oct 2003 13:28:17 +0000, Shi Jin wrote:

>
> The question is how is broadcast actually done when the masternode tries
> to broadcast data to all the other nodes? I am not a computer scientist
> but I kind of know that on a hub any message is broadcast on the same
> network and everybody could get it. While for my understanding, for
> switch, there is no way do such kind of broadcasting. So although in
> most cases, a switch is suprior to a hub but if what I need is to
> broadcast large amout of data quickly through the local network, maybe
> switch is not as good as a hub. Is that right?
>

To begin with a VLAN (Virtual Local Area Network) basically defines your
broadcast domian - Whicn means the switch would only forward broadcast
frames out ports where there are other members of the same VLAN. That
being said, if your switch supports VLANs, by default all ports are in
VLAN 1 so broadcast frames are flooded out all ports except for
the one port the broadcast frame entered the switch - in effect the same
as action a hub. Most all switches today operate in the "store and
forward" mode (receive the entire frame before forwarding) while most hubs
regenerate the frame in a bit stream manner so there is higher latency
through a switch than a hub. This is offset by the fact that unicast
frames are only forwarded out the port where the destination MAC address
resides allowing multiple frames going to different destinations to be
forwarded at the same time where as a hub can handle only 1 frame at a
time. So to answer your last question, if your communications is more of
a master-slave type with the "Master controller" broadcasting to the
secondaries with little "peer to peer" or slave to master communications a
hub "Might" be better solution. If you have a mix of traffic the switch
would probably provide better performance

Hows that for a consultants' answer (could have been reduced to 'It
depends'

On Fri, 10 Oct 2003 11:50:48 +0100, David Haggett
<news-(E-Mail Removed)> wrote:

>On Fri, 10 Oct 2003 13:34:41 +1000, Leon The Peon wrote:
>
>It might actually broadcast the data (i.e. use a destination address of
>ff:ff:ff:ff:ff:ff in the ethernet frame). This is a very efficient way of
>distributing common information if ALL HOSTS on the segment need to hear
>it.
Efficient in the meaning that you reach all your recipients with one
address. However, any other host on the network will also receive it.
Because it is a broadcast, this means that all everybody must pick up
the frame and decode it to see if it is for them. This is not
necessarily a problem, but certainly not efficient communication when
looking at the whole picture (actually it reminds quite a bit about
spam mail: send it to everybody, and you probably reach someone who
appreciates it. The rest though...)

>Alternatively it might use multicast, a special mode of broadcast which is
>targeted at a specific group of hosts. This is only efficient if the
>switch infrastructure understands multicast and stops it from reaching
>hosts that don't want it.
Even if your switch lack the necessary features to prune multicast
traffic, you would still avoid bothering hosts not listening for the
specific multicast address.

This might of course be totally irelevant if all the host on the
network are supposed to be recipiants of the traffic :-)

Regards,
Geir

Shi Jin wrote:

> Hi there,
>
> I know this is a little off topic but I am not sure where I shoud post
> this question. Any hint where I should post is welcome.
>

So, who's this broad you're talking about? ;-)


--

Fundamentalism is fundamentally wrong.

To reply to this message, replace everything to the left of "@" with
james.knott.

On Thu, 9 Oct 2003 17:33:15 +0000 (UTC), Frank Sweetser <(E-Mail Removed)>
wrote:
>(VLANs, BTW, have nothing to do with
>what you're asking about)

Well, they can, if it's a large switch covering multiple virtual networks.
The VLAN setup would guarantee that a subnetted broadcast from a
particular VLAN would only go out to machines on that VLAN.


--
#include <standard.disclaimer>
_
Kevin D Quitt USA 91387-4454 96.37% of all statistics are made up
Per the FCA, this address may not be added to any commercial mail list

Fellows every time I see this subject I think:
"Broads"=women
"switch"=well lots of things, one usage is a person who
can act as either dominant or submissive in S&M.

-Vincent "prevert" Fox

--
Vincent Fox
Georgia Institute of Technology, Atlanta Georgia, 30332
uucp: ...!{decvax,hplabs,ncar,purdue,rutgers}!gatech!pri sm!vf5
Internet: (E-Mail Removed)