Ports 1 & 2 are not oversubscribed so you get the full 1Gbps on each port.
Then the rest of the ports are placed into groups of 4 and each group of 4 ports has a bandwidth capacity of 1Gbps wire speed.
This means that if you have 4 machines connected into a group of 4 ports and each machine was running at 100 Fast ethernet there would be no oversubscription.
But if all the machines are running 1Gbps NIC's there is the possibility for oversubscription. The possibility is entirely dependant on how much data each server actually pumps out.
The port groupings for your module are
3 5 7 9
4 6 8 10
11 13 15 17
12 14 16 18
Attached is a link to a doc that explains it in more detail together with an example of how you can distribute your servers across the module.
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