[techtalk] router/switch question

[Raven, corporate courtesan]

Heya --

Quoth Tania M. Morell (Wed, Feb 06, 2002 at 12:18:40AM -0500):
> No, I'm sure it's a switch and not a hub.  ;)   This activity thing doesn't
> happen with any of my other computers so I think it may have something to do
> with the network config on this particular machine but I don't know enough
> about networks/ips/broadcasts/netmasks/etc to know right off what it is. I
> guess I'll have to tinker around until I figure it out.
 
	It may be the app, too.  (I forget what you said you were doing
when you saw all that activity.)  Can you run the app on a different
machine and see if you see the same behaviour?  If so, the app is
probably just one that uses a lot of broadcasts, and that's why
you're seeing that activity everywhere.

	Switches 101:  Switches are often called smart hubs.  This is
because they learn where devices are (Mac address/IP address blah is off
port foo) by listening to frames they recieve.  So, if they get a frame
from MAC address foo on port 1 destined for MAC address bar, they store
the knowledge that MAC address foo is off port 1 in their forwarding
table.  The next time they have a frame for foo, they can only send it
out of port 1 rather than spamming all ports with it.  (If switches
recieve a frame for an unknown MAC address, they'll generally forward it
out all ports except the one it came in on.)  As they pass frames, they
learn where the devices on the local network are, and they send less
frames to all ports, and more for the particular port that connects to
the destination device.

	The exception to this -- broadcast frames.  These are frames
that are *supposed* to go to everyone on the local subnet (a division of
IP space).  So broadcast frames will appear on every port except the one
they came in on, every time.  It sounds like this is what's happening
with your setup.  Broadcast frames have many uses for protocol designers
-- find all machines on the local network, send a message to all (Router
going down in five minutes!), search for a particular computer that you
know is connected but don't know where it is (akin to shouting "Hello?
Jane?" down every hallway until she shows up), aanouncing available
services to other devices on a network (a la Novell servers and
AppleTalk printers), things like that.

	Some applications are more broadcast-intensive than others.  It
all depends on the software, the protocol stack, and how it was
programmed.

	If any of this doesn't make sense, please ask.

Cheers,
Raven
 
 
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