switch speed question

Wed Feb 25 15:16:23 UTC 2009

Nathan Ward wrote:
> On 26/02/2009, at 2:48 AM, David Barak wrote:
>> If two hosts are exchanging 1Gbps flows, the traffic across the bus
>> will be 2Gbps, right?
>
> You don't get to add transmit and receive together to get 48Gbps.
> Packets don't go across the backplane once to receive, and then once
> more to transmit. They go across once, from the receiving port to the
> transmitting port. (sure, sometimes perhaps packets do go across
> twice, but not normally)

Assuming a crossbar switch, sure.  If your ports individually look up
the outgoing port for an incoming packet, request backplane to that
port, and transmit, then you only need 24Gbps.  If your ports need to
connect to an intelligent entity on the backplane to do your
routing/switching/IGMP snooping/QoS enforcement/etc, then you are indeed
going to cross the backplane twice, and need both transmit and receive
bandwidth.

Since many of us are routing goons with store-and-forward roots, we tend
to think along those lines.  And it is still wise, even in this day and
age, to make sure that backplane bandwidth doesn't include a central
switching point, or, if it doesn't, the marketing folks haven't doubled
the backplane numbers because they took it out.

-Dave