I didn't see this posting until after my prior posting, and yes, it confirms each port group is limited to a gig. (Actually I thought that too might be the case, but wasn't sure, so my prior posting didn't address the actual bandwidth limitation. This reference further documents this in one of its restriction sections, see below.)
The WS-X6148A-GE-TX switching module has 48 oversubscribed ports in six groups of eight ports each:
Ports 1, 2, 3, 4, 5, 6, 7, 8
Ports 9, 10, 11, 12, 13, 14, 15, 16
Ports 17, 18, 19, 20, 21, 22, 23, 24
Ports 25, 26, 27, 28, 29, 30, 31, 32
Ports 33, 34, 35, 36, 37, 38, 39, 40
Ports 41, 42, 43, 44, 45, 46, 47, 48
The eight ports within each group use common circuitry that effectively multiplexes the group into a single, nonblocking, full-duplex Gigabit Ethernet connection to the internal switch fabric. For each group of eight ports, the frames that are received are buffered and sent to the common Gigabit Ethernet link to the internal switch fabric. If the amount of data received for a port begins to exceed buffer capacity, flow control sends pause frames to the remote port to temporarily stop traffic and prevent frame loss.
If the frames received on any group exceeds the bandwidth of 1 Gbps, the device starts to drop the frames. These drops are not obvious as they are dropped at the internal ASIC rather than the actual interfaces. This can lead to slow throughput of packets across the device
If more throughput is required, either use ports from a line module that does not use oversubscription or use ports from different port-groupings on oversubscribed line modules. For example, if the line module has 48 ports in groups of eight, you can select ports 1, 9, 17, 25, 33, and 41 for the same port channel.
Cisco recommends that you use 61xx, 63xx, and 64xx modules for access layer as they are usually oversubscribed and always have only a bus connection to the backplane switching bus. For a backbone connection, it is recommended that you use 65xx or 67xx modules which always have either an eight or 20 GB dedicated connection to the switch fabric.
