| Featured Functions | ||||
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| Enable STP per Port |
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| Setting | Description | Factory Default |
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| Enable/Disable | Select to enable the port as a node on the | Disabled |
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| Spanning Tree topology. |
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| NOTE | We suggest not enabling the Spanning Tree Protocol once the port is connected to a device (PLC, | |||
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| RTU, etc.) as opposed to network equipment. The reason is that it will cause unnecessary | |||
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| negotiation. |
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| Port Priority |
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| Setting | Description | Factory Default |
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| Numerical value selected | Increase this port’s priority as a node on the | 128 |
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| by user | Spanning Tree topology by inputting a lower |
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| number. |
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| Port Cost |
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| Setting | Description | Factory Default |
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| Numerical value input by | Input a higher cost to indicate that this port is | 200000 |
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| user | less suitable as a node for the Spanning Tree |
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| topology. |
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Port Status
Indicates the current Spanning Tree status of this port. “Forwarding” for normal transmission, or “Blocking” to block transmission.
Configuration Limits of RSTP/STP
The Spanning Tree Algorithm places limits on three of the configuration items described above:
[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)
MOXA
Hello Time = 5 sec, Max. Age = 20 sec, and Forwarding Delay = 4 sec does not violate Eqs. 1 through 3, but does violate Eq. 4, since in this case,
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 sec.
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.
