switch speed question

Nathan Ward nanog at daork.net
Wed Feb 25 14:15:27 UTC 2009

On 26/02/2009, at 2:48 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?

Yes. 1Gbps backplane impact per host. You have two hosts, right? One  
host per port? That's 1Gbps per port.
So, 24 ports = 24Gbps, right?

Let's try look at it another way:
- A 24 port gig switch can receive at most 24Gbps.
- That same switch can transmit at most 24Gbps.

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)

> 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.

Of course multicast/broadcast consumes more resources than the input  
rate. That's the point. If you receive multicast or broadcast at  
1Gbps, and the multicast needs to go out all the ports, you need to  
transmit at 24Gbps. That's 24 x the transmit resources (and probably  
backplane resources, depending on architecture etc. etc.) than a  
single 1Gbps unicast stream.

Of course, with unicast it is only getting to one host.

Let's assume we have data at 1Gbps that we need to get to 24 hosts.
- If we unicast, we need 24 input ports, and 24 output ports, assuming  
we only have gig ports (or say 3x10GE, or whatever).
- If we multicast, we need 1 input port, and 24 output ports.

When you compare the end result, multicast uses significantly less  
resources, right?

In fact, perhaps some bus architectures know about how multicast  
works, and it consumes *less* resources than doing the same thing with  
many unicast streams. If the bus does not know about multicast, then  
the bus would treat it as 24 unicast streams, surely.

Nathan Ward

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