D-Link DES-1228/ME User Manual - Stp Loopback Prevention
DES-1228/ME Layer 2 Fast Ethernet Managed Switch
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STP LoopBack Prevention
When connected to other switches, STP is an important configuration in consistency for delivering packets to ports and can
greatly improve the throughput of your switch. Yet, even this function can malfunction with the emergence of STP BPDU packets
that occasionally loopback to the Switch, such as BPDU packets looped back from an unmanaged switch connected to the DES-
1228/ME. To maintain the consistency of the throughput, the DES-1228/ME now implements the STP LoopBack prevention
function.
When the STP LoopBack Detection function is enabled, the Switch will be protected against a loop occurring between switches.
Once a BPDU packet returns to the Switch, this function will detect that there is an anomaly occurring and will place the receiving
port in an error-disabled state. Consequentially, a message will be placed in the Switch’s Syslog and will be defined there as
“BPDU Loop Back on Port #”.
Setting the LoopBack Timer
The LoopBack timer plays a key role in the next step the switch will 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 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 versions of STP (STP and RSTP) can enable this feature.
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May be configured globally (STP Global Bridge Settings).
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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 LoopBack timer is 60 seconds.
•
This setting will only be operational if the interface is STP-enabled.
The LoopBack 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 LoopBack condition on the elected root port of STP on another
switch
STP Bridge Global Settings
To view the STP Bridge Global Settings window, click L2 features > Spanning Tree > STP Bridge Global Settings.