http://www.cisco.com/en/US/docs/switches/datacenter/mds9000/sw/4_1/configuration/guides/cli_4_1/sys.html#wp1195879
Performing Internal Loopback Tests
You can run manual loopback tests to identify hardware errors in the data path in the switching or services modules, and the control path in the supervisor modules. Internal loopback tests send and receive FC2 frames to/from the same ports and provide the round trip time taken in microseconds. These tests are available for Fibre Channel, IPS, and iSCSI interfaces.
Use the EXEC-level system health internal-loopback command to explicitly run this test on demand (when requested by the user) within ports for the entire module.
switch# system health internal-loopback interface iscsi 8/1
Internal loopback test on interface iscsi8/1 was successful.
Sent 1 received 1 frames
Round trip time taken is 79 useconds
Use the EXEC-level system health internal-loopback command to explicitly run this test on demand (when requested by the user) within ports for the entire module and override the frame count configured on the switch.
switch# system health internal-loopback interface iscsi 8/1 frame-count 20
Internal loopback test on interface iscsi8/1 was successful.
Sent 1 received 1 frames
Round trip time taken is 79 useconds
Use the EXEC-level system health internal-loopback command to explicitly run this test on demand (when requested by the user) within ports for the entire module and override the frame length configured on the switch.
switch# system health internal-loopback interface iscsi 8/1 frame-count 32
Internal loopback test on interface iscsi8/1 was successful.
Sent 1 received 1 frames
Round trip time taken is 79 useconds
