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STP Loopback Detection, Setting the Loopback Timer - reset

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DGS-3024 Gigabit Ethernet Switch Manual STP Loopback Detection 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 loop back to the Switch, such as BPDU packets looped back from an unmanaged switch connected to the DGS3024. To maintain the consistency of the throughput, the DGS-3024 switch implements the STP Loopback Detection 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 Loopback 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 • All three versions of STP (STP, RSTP and MSTP) can enable this feature. • May be configured globally (STP Global Bridge Settings), or per port (MSTP Port Information). • Neighbor switches of the DGS-3024 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 DGS-3024 switch. • Loopback Detection is globally enabled for the Switch, yet the port-by-port default setting 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 the DGS-3024 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. 39

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DGS-3024 Gigabit Ethernet Switch Manual
STP Loopback Detection
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 loop back to the Switch, such as BPDU packets looped back from an unmanaged switch connected to the DGS-
3024. To maintain the consistency of the throughput, the DGS-3024 switch implements the STP Loopback Detection 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 Loopback 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
All three versions of STP (STP, RSTP and MSTP) can enable this feature.
•
•
•
•
•
•
May be configured globally (STP Global Bridge Settings), or per port (MSTP Port Information).
Neighbor switches of the DGS-3024 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 DGS-3024 switch.
Loopback Detection is globally enabled for the Switch, yet the port-by-port default setting 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 the DGS-3024 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.
39