D-Link DES-3528 - xStack Switch - Stackable Reference Manual page 93

xStack® DES-3528/DES-3552 Series Layer 2 Stackable Fast Ethernet Managed Switch Web UI Reference Guide
802.1D-1998/802.1D-2004/802.1Q-2005 Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to
802.1D-1998 format when necessary. However, any segment using 802.1D-1998 STP will not benefit from the rapid
transition and rapid topology change detection of MSTP or RSTP. The protocol also provides for a variable used for
migration in the event that legacy equipment on a segment is updated to use RSTP or MSTP.
The Spanning Tree Protocol (STP) operates on two levels:
1. On the switch level, the settings are globally implemented.
2. On the port level, the settings are implemented on a per-user-defined group of ports basis.
BPDU Loop-back 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 Switch. To maintain the consistency of the throughput, the Switch now implements the BPDU
Loop-back prevention function.
When the BPDU Loop-back 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 Loop-back Timer
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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