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I've always wondered if the bandwidth rating of a switch is per port, or total capacity? For example, if I have PC1 and PC2 that are capable of saturating a 100 Mb connection plugged into a 10/100 switch, if I add PC3 and PC4 to the same switch and start transferring files from PC1 to PC2 and from PC3 to PC4 - will both sets of PC's have 100 Mb to use?

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up vote 7 down vote accepted

The 100Mbps speed of a switch port refers to the max one-way line speed of the port.

If you have two PCs on the switch and they are connected at 100Mbps full-duplex, you actually have 200Mbps of bandwidth between them - 100Mbps from A to B, and 100Mbps from B to A. (Some switch/NIC vendors take advantage of this on the packaging - don't be fooled!)

Switches are also limited on the backplane - they can typically handle up to a certain amount of bandwidth in terms of Mbps as well as a maximum number of PPS (packets per second).

Most of the time, you don't need to worry about saturating the backplane - you won't.

In your example, the traffic you're generating from PC1 to PC2 will go at max 100Mbps and the traffic from PC3 to PC4 will go at max 100Mbps - they won't interfere with each other.

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You can saturate the backplane in some odd cases like if you have a bunch of servers that are basically network IO bound talking to each other on the same switch. IIRC some switches can handle full speed traffic on every port, some can't. But as was pointed out, in general you will never run /all/ the ports near the limit. – BCS May 25 '09 at 18:57

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