D-Link Xstack DES-3528 Series Reference Manual page 101

xStack® DES-3528/DES-3552 Series Layer 2 Managed Stackable Fast Ethernet Switch Web UI Reference Guide
The Loop-back timer plays a key role in the next step for the Switch to take to resolve this problem. Choosing a
non-zero value on the timer will enable the Auto-Recovery Mechanism. When the timer expires, the Switch will
again look for its returning BPDU packet on the same port. If no returning packet is received, the Switch will recover
the port as a Designated Port in the Discarding 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 Loop-back 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 Loop-back Recover Time is
set to 0.
Regulations and Restrictions for the Loop-back Detection Function
• All versions of STP (STP and RSTP) can enable this feature.
• May be configured globally (STP Global Bridge Settings).
• Neighbor switches of the Switch 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 Switch.
• The default setting for this function is disabled.
• The default setting for the Loop-back timer is 60 seconds.
• This setting will only be operational if the interface is STP-enabled.
The Loop-back Detection feature can only prevent BPDU loops on designated ports. It can detect a loop condition
occurring on the user's side connected to the edge port, but it cannot detect the Loop-back condition on the elected
root port of STP on another switch
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