Moxa Technologies Moxa EtherDevice Switch Configuration Limits of RSTP/STP, Port Priority

EDS-510A Series User’s Manual		Featured Functions
NOTE	We suggest not enabling the Spanning Tree Protocol once the port is connected to a device (PLC,
	RTU, etc.) as opposed to network equipment. The reason is that it will cause unnecessary
	negotiation.
	Port Priority
	Setting	Description	Factory Default
	Numerical value selected	Increase this port’s priority as a node on the	128
	by user	Spanning Tree topology by entering a lower
		number.
	Port Cost
	Setting	Description	Factory Default
	Numerical value input by	Input a higher cost to indicate that this port is	200000
	user	less suitable as a node for the Spanning Tree
		topology.

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 previously:

[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 EDS-510A’s firmware will alert you immediately if any of these restrictions are violated. For example, setting

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 many ways. One solution is simply to increase the Forwarding Delay value to at least 11 sec.

HINT: Perform 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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