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I was recently told that if I had two switches connected to one another
through an uplink port connected at gigabit speeds that the traffic
flow dynamically changed as different devices connected to other ports
on the two switches communicated. What I mean is that if I have a
10mbit printer on switch 1 talking to a 100 mbit PC on switch 2 through
the gigabit uplink, the gigabit link actually changes to 10 mbit for
the time the traffic is transmitting. Is this an accurate
representation of what is happening or does the switch handle traffic
flow on a per port speed basis?

Thanks

Tom

It isn't even close to true.

All moderns switches use a "store and forward" architecture. They
receive a packet at whatever speed (e.g., 10 mbit) and store the
entire packet, and then release the packet into the internal bus.
The internal bus transmits at the maximum internal speeds supported
by the various internal hops in the architecture, until the packet
logically reaches the destination port in the same switch
(or switch "cluster"). The internal hardware then waits until the
destination port is available, and when it is, transmits at whatever
speed the destination port supports.

In your example, switch 1 would receive the printer packet at 10 Mbit/s,
shuffle it internally, and queue it up for the 1 gigabit uplink
that is the destination as far as switch 1 is concerned. The packet
would go over the uplink at 1 gigabit as soon as there is an opportunity,
where it would be received at switch 2 at 1 gigabit. Switch 2 would
then shuffle the packet internally, and queue it up for the
port for the PC, transmitting it at 100 Mbit/s as soon as there is an
opportunity on that port.
--
"law -- it's a commodity"
-- Andrew Ryan (The Globe and Mail, 2005/11/26)

tom@tek-3.com wrote:

I was recently told that if I had two switches connected to one another
through an uplink port connected at gigabit speeds that the traffic
flow dynamically changed as different devices connected to other ports
on the two switches communicated. What I mean is that if I have a
10mbit printer on switch 1 talking to a 100 mbit PC on switch 2 through
the gigabit uplink, the gigabit link actually changes to 10 mbit for
the time the traffic is transmitting. Is this an accurate
representation of what is happening or does the switch handle traffic
flow on a per port speed basis?

Gigabit is gigabit--any time data is moving over it it is moving at the
gigabit rate. If the devices attached to the switch cannot accept data at
the rate that it is coming into the switch, the switch may generate pause
frames or may just drop frames for which it does not have room in its
buffer--both modes of operation are consistent with the Ethernet standards.
Usually any throttling takes place at a level above that of
Ethernet--TCP/IP detects that frames are being dropped and adjusts its
behavior accordingly.

You'll find a bit of information on this at
<http://en.wikipedia.org/wiki/Ethernet_flow_control>.

Thanks

Tom

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

tom@tek-3.com wrote:

I was recently told that if I had two switches connected to one another
through an uplink port connected at gigabit speeds that the traffic
flow dynamically changed as different devices connected to other ports
on the two switches communicated. What I mean is that if I have a
10mbit printer on switch 1 talking to a 100 mbit PC on switch 2 through
the gigabit uplink, the gigabit link actually changes to 10 mbit for
the time the traffic is transmitting. Is this an accurate
representation of what is happening or does the switch handle traffic
flow on a per port speed basis?

One switch port has absolutely no effect on another. Each port will
independently set it's speed to match what's on the other end of the cable.

Thank you all for your knowledge.

Tom