PT-7728 User’s Manual Featured Functions
3-40
NOTE
We suggest that you disable the Spanning Tree Protocol for ports that are connected directly to a
device (PLC, RTU, etc.) as opposed to network equipment. This will prevent unnecessary
negotiation.
Port Priority
Setting Description Factory Default
Numerical value selected
by user
This specifies the port’s priority as a node on
the Spanning Tree topology. Lower values
correspond to higher priority.
128
Port Cost
Setting Description Factory Default
Numerical value input by
user
This specifies the port cost. Higher costs
correspond to lower suitability as a node for the
Spanning Tree topology.
200000
Port Status
Indicates the current Spanning Tree status of this port. Forwarding indicates normal transmission
and Blocking indicates blocked transmission.
Configuration Limits of RSTP/STP
The Spanning Tree Algorithm places limits on three of the configuration items:
[Eq. 1]: 1 sec Hello Time 10 sec≦≦
[Eq. 2]: 6 sec Max. Age≦ 40 sec≦
[Eq. 3]: 4 sec Forwarding Delay 30 sec≦≦
These three variables are further restricted by the following two inequalities:
[Eq. 4]: 2 * (Hello Time + 1 sec) Max. Age 2 * (Forwarding Delay ≦≦ – 1 sec)
The PT-7728’s firmware will alert you immediately if any of these restrictions are violated. For
example, suppose Hello Time = 5 sec, Max. Age = 20 sec, and Forwarding Delay = 4 sec. This
does not violate Eqs. 1 through 3, but it violates Eq. 4:
2 * (Hello Time + 1 sec) = 12 sec, and 2 * (Forwarding Delay – 1 sec) = 6 sec.
You can remedy the situation in any number of ways. One solution is simply to increase the
Forwarding Delay value to at least 11 seconds.
HINT: Take the following steps to avoid guessing:
Step 1: Assign a value to “Hello Time” and then calculate the left most part of Eq. 4 to get the
lower limit of Max. Age.
Step 2: Assign a value to “Forwarding Delay” and then calculate the right most part of Eq. 4 to
get the upper limit for Max. Age.
Step 3: Assign a value to Forwarding Delay that satisfies the conditions in Eq. 3 and Eq. 4.
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