17.1.2 Operating specifications

In L2 loop detection, an L2 control frame for detecting an L2 loop (L2 loop detection frame) is sent regularly from the port (a physical port or a channel group) specified in the configuration section. If the L2 loop detection frame is received on a port on which the L2 loop detection functionality is enabled, a loop failure is detected, and the port on which the frame is received or the port originating the frame is deactivated.

After the cause of the loop failure has been corrected, an operation command can be used to activate the deactivated port. If the automatic-restoration functionality has been configured, the deactivated port can be activated automatically.

<Structure of this section>

(1) Port type for L2 loop-detection function

The following table describes the types of ports used by the L2 loop detection functionality.

Table 17-1: Port types
Type	Functionality
Detecting and blocking port	This port sends an L2 loop detection frame to detect a loop. If a loop failure is detected, an operation log is displayed and the problem port is deactivated by this port.
Detecting and sending port	This port sends an L2 loop detection frame to detect a loop. If a loop failure is detected, an operation log is displayed. The problem port is not deactivated.
Detecting port (when configuration has not been performed)	This port does not send an L2 loop detection frame to detect a loop. If a loop failure is detected, an operation log is displayed. The problem port is not deactivated.
Ports exempted from detection	Any port for which the functionality is not used. The L2 loop detection frame for detecting a loop is not sent and a loop failure is not detected.
Uplink port	This port does not send an L2 loop detection frame to detect a loop. If a loop failure is detected, an operation determined by the port type of the source port is performed. For example, if the source port is a detecting and blocking port, an operation log is displayed, and the source port is deactivated.

(2) About L2 Loop-Detect Frame Transmitter

An L2 loop detection frame is sent from all VLANs belonging to the detecting and blocking port and the detecting and sending port within the specified interval. The maximum number of frames that can be sent with the functionality is predetermined, and any frames exceeding the maximum are not sent. In addition, loop failures will no longer be able to be detected on ports or the VLANs from which the frames could not be sent. For this reason, specify a maximum number of frames according to the capacity limits. For more information, see the Configuration Guide Vol.1" "3.9.1 L2 Loop Detection.

(3) How to detect looping failures and when to place a port in inactive status

When a L2 loop detection frame is received, a loop failure is regarded even between VLAN if it is a L2 loop detection frame sent from the Switch and VLAN is set for the receive port. When a loop failure is determined by reception of a L2 loop detection frame, the number of frames received is counted for each port. When this value reaches L2 loop-detection frame receive count (the default value is 1) set in the configuration, the corresponding port is turned inactive.

(4) L2 Loop-Detecting Behavior in a Stacked Configuration

Even if L2 loop detection frames sent between member switches within the same stack are received, the L2 loop detection functionality is operational. The following figure shows loop detection in response to L2 loop frames in a stack configuration.

Figure 17-2: Loop detection when a L2 loop detection frame is received during stacking

(5) Displaying operation messages

Even if an operation message for loop failure detection is displayed on any port and a L2 loop detection frame is received on the same port immediately after that, the operation message is not displayed for one minute from the previous display. If a L2 loop detection frame is received after one minute has elapsed since the previous display, a loop failure detection operation message is displayed.