Switch Bandwidth

Unanswered Question
Jan 15th, 2009

Is this accurate...

If I have a switch (3560) with 48 - 10/100 Mbps ports and 1 - SFP Module (1000base-T) for the trunk..

The throughput from one device to another on the switch is limited to 100 Mbps. The trunk line is limited to 1 Gbps. So if all 48 ports are sending data to a device outside the switch (through through the trunk port) the maximum throughput is limited to 1 Gbps that is my trunk port. So 1000 Mbps / 48 ports = 20.83 Mbps per port. So if all 48 ports were transmitting to full capacity they would only get 20.83 Mbps each?


I have this problem too.
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Jon Marshall Thu, 01/15/2009 - 12:16


Yes in essence if all devices were transmitting to another device which was not local to the switch the limiting factor would be the 1GBps uplink. Whether or not all ports got 20.83Mbps is unlikely. Bear in mind very few devices can transmit at 100Mbps consistently.

What we are talking about here is oversubscription. Oversubscription is part of designing a network because it would be a very rare occurence where all ports are transmitting at exactly the same time. Based on application traffic, traffic patterns ie. is a lot of the traffic local to the switch or is remote, this is a large part of device selection.

Yes the easiest thing is to try and have no oversubscription but the cost is usually prohibitive hence the compromise.


Joseph W. Doherty Fri, 01/16/2009 - 11:56

Since the 48 port 3560s have four SFP ports, also keep in mind you might have the option to use those additional ports for additional bandwidth. With all in operation, you have enough aggregate bandwidth for forty 100 Mbps ports. (Jon is correct, it's unlikely you would need that much bandwidth. Some bandwidth oversubsciption is very common.)


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