What does the speed of Network Switch actually mean?

Question

Network switches usually come in speeds of 10/100/1000 Mbps, and that's all fine and dandy, then I got thinking..

Suppose I have an 8 port 1000 Mbps switch, each port connected to a client. If a computer on port (lets say 3) is sending data at 1000 Mbps to a client on port 8, does that saturate all the available bandwidth for clients on other ports? Let's say at the same time another client (port 6) which is capable of 1000 Mbps transfers is about to start talking to the NAS on port 5. Are both data transactions going to half in speed?

Thanks in advance.

Answer 1

Traffic on a switch is sent directly from host interface to host interface. You could certainly have have multiple interfaces running at full speed that overload a single interface that is the same speed as each of the other interfaces. That is called oversubscription.

More directly to your question, a stream of traffic between two interfaces normally has no effect on a stream of traffic between two other interfaces.

It is not usual for a PC to be transmitting at full speed for an extended period of time, although that may be true for a server. Oversubscription usually happens between multiple access (host) interfaces and an uplink, but it is not such a problem as you may imagine. Cisco recommends a 20:1 access to uplink bandwidth ratio.