D-Link Xstack DGS-3600 Series User Manual page 134

State. If another returning BPDU packet is received, the port will remain in a blocked state, the timer will reset to the specified
value, restart, and the process will begin again.
For those who choose not to employ this function, the Loopback Recovery time must be set to zero. In this case, when a BPDU
packet is returned to the Switch, the port will be placed in a blocking state and a message will be sent to the Syslog of the switch.
To recover the port, the administrator must disable the state of the problematic port and enable it again. This is the only method
available to recover the port when the Loopback Recover Time is set to 0.
Regulations and Restrictions for the Loopback Detection Function
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All three versions of STP (STP, RSTP and MSTP) can enable this feature.
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May be configured globally (STP Global Bridge Settings), or per port (MSTP Port Information).
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Neighbor switches of the xStack DGS-3600 Series switches must have the capability to forward BPDU packets. Switches
the fail to meet this requirement will disable this function for the port in question on the xStack DGS-3600 Series switches.
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Loopback Detection is globally enabled for the switch, yet the port-by-port default setting is disabled.
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The default setting for the Loopback timer is 60 seconds.
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This setting will only be operational if the interface is STP-enabled.
The Loopback Detection feature can only prevent BPDU loops on the xStack DGS-3600 Series switches designated ports. It can
detect a loop condition occurring on the user's side connected to the edge port, but it cannot detect the Loopback condition on the
elected root port of STP on another switch.
xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch
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