H3C Technologies H3C S12500-X Series Switches - Manual (Page 17)

6
Performing a loopback test on an Ethernet interface
If an Ethernet interface does not work correctly, you can perform a loopback test on it to identify the
problem. An Ethernet interface in a loopback test does not forward data traffic.
Loopback tests include the following types:
• Internal loopback test—Tests all on-chip functions related to Ethernet interfaces.
• External loopback test—Tests the hardware of Ethernet interfaces. To perform an external loopback
test on an Ethernet interface, connect a loopback plug to the Ethernet interface. The device sends
test packets out of the interface, which are expected to loop over the plug and back to the interface.
If the interface fails to receive any test packets, the hardware of the interface is faulty.
Configuration restrictions and guidelines
• On an administratively shut down interface (displayed as in ADM or Administratively DOWN state),
you cannot perform an internal or external loopback test.
• The speed, duplex, and shutdown commands are not available during a loopback test.
• During a loopback test, the Ethernet interface operates in full duplex mode. When a loopback test
is complete, the port returns to its duplex setting.
Configuration procedure
To perform a loopback test on an Ethernet interface:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enter Ethernet interface view. interface interface-type
interface-number
N/A
3. Perform a loopback test. loopback { external | internal }
By default, no loopback test is
performed.
Configuring generic flow control on an Ethernet interface
IMPORTANT:
Enabling or disabling flow control on an interface might shut down and then bring up the interface. Make
preparations before enabling or disabling this feature.
To avoid packet drops on a link, you can enable generic flow control at both ends of the link. When
traffic congestion occurs at the receiving end, the receiving end sends a flow control (Pause) frame to ask
the sending end to suspend sending packets.
• With TxRx mode generic flow control enabled, an interface can both send and receive flow control
frames. When congestion occurs, the interface sends a flow control frame to its peer. When the
interface receives a flow control frame from the peer, it suspends sending packets.
• With Rx flow mode generic control enabled, an interface can receive flow control frames, but it
cannot send flow control frames. When the interface receives a flow control frame from its peer, it
suspends sending packets to the peer. When congestion occurs, the interface cannot send flow
control frames to the peer.