Hi

Restrictions.

This section contains usage guidelines, restrictions, and troubleshooting information that apply to EtherChannel:

1. The WS-X6548-GE-TX, WS-X6548V-GE-TX, WS-X6148-GE-TX, and WS-X6148V-GE-TX modules have a limitation with EtherChannel. EtherChannel is supported on these modules for all configurations (10, 100, and 1000 Mbps speeds) but be aware of these cases of oversubscription when you configure these modules:

   1. On these modules there is a single 1-Gigabit Ethernet uplink from the port ASIC that supports eight ports. For EtherChannel, the data from all links in a bundle goes to the port ASIC, even though the data is destined for another link. This data consumes bandwidth in the 1-Gigabit Ethernet link. For these modules, the sum total of all data on an EtherChannel cannot exceed 1 Gigabit.

      You receive a message on the maximum throughput when you add a port of this module to EtherChannel.

      C6500> (enable) set port channel 3/5,4/5 mode on
      Adding a WS-X6148-GE-TX port to a channel limits the channel's
      bandwidth to a maximum of 1Gig throughput
      Port(s) 3/5,4/5 channel mode set to on.
      C6500> (enable)

   2. You can also run into the oversubscription problem if you have four WS-X6148-GE-TX or WS-X6148V-GE-TX modules that run at 100 Mbps with 48 EtherChannels, and each channel has 4 ports (1 port per module).

   3. If you use the Switch Fabric Module with the WS-X6548-GE-TX or WS-X6548V-GE-TX modules, that configuration avoids the oversubscription problem. The Switch Fabric Module interface filters and distributes the packets to the correct module per the EtherChannel bundle hash. But, you must have one port per module in the bundle. Once you have more than one port of a WS-X6548-GE-TX or WS-X6548V-GE-TX module in an EtherChannel bundle it starts to oversubscribe.

      Note: With Catalyst OS software release 8.2(1), due to firmware enhancements, these oversubscription problems are no longer an issue with the WS-X6548-GE-TX and WS-X6548V-GE-TX modules.

   Refer to Ethernet and Gigabit Ethernet Switching Modules - Catalyst 6500 Series Switch Module Guide for the list of other 10/100/1000 Ethernet Switching modules and Gigabit Ethernet Switching modules.

2. When you enable UplinkFast, the EtherChannel port path cost, which you set with the set channel cost command, for a 4-port 10/100 EtherChannel is less than the port path cost of a parallel Gigabit Ethernet link. This situation causes the slower 4-port EtherChannel to forward and the Gigabit Ethernet link to block. Workaround is to explicitly configure a higher cost for the channel after you enable UplinkFast. Cisco bug ID CSCds22895 ( registered customers only) tracks this issue.

3. 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.

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml

Regards

Chetan kumar