switch speed question

Tom Storey tom at snnap.net
Wed Feb 25 14:13:01 UTC 2009

Were not considering anything other than basic switching in this  
scenario, as is my understanding.

2 hosts will create 2gbps of traffic as each host is inputting 1gbps  
into the switch (just multiply it by 12 to give you 24 ports). 3 hosts  
will create 3gbps of traffic as each inputs 1gbps into the switch  
(e.g. each host could be sending 500mbps to each of the other hosts).  
And thus and so forth. :-)

You can only input a maximum of 24gbps into the switch, which means  
that only 24gbps will cross the backplane.

Yes there is 48gbps if you combine tx and rx of each port, but traffic  
only has to cross the backplane once, from rx on one port to tx on  

Sorry if I have hijacked this thread from the OP. :-)


On 26/02/2009, at 12:18 AM, David Barak wrote:

> Doesn't that assume that the communicarion is unidirectional?
> If two hosts are exchanging 1Gbps flows, the traffic across the bus  
> will be 2Gbps, right?
> And of course, this doesn't include any bus-intensive operations  
> like multicast
> or things which require cpu processing - those can consume a lot  
> more resources than the input rate of the port.
> -David Barak
> Tom Storey wrote:
>>> Not every bit in results in just one bit out.  Broadcast, multicast,
>>> flooding for unknown MACs (or switching failures), ...
>> They were talking about a simple scenario where a bit that enters a  
>> port
>> will leave a port. With 24 gigabit ports, for all intents and  
>> purposes,
>> you will only ever have 24 gigabits at the most traversing the  
>> backplane.

More information about the NANOG mailing list