Moxa Technologies 405A, EDS-408A Configuration Limits of RSTP/STP, Blocking, Port Cost, Forwarding, Port Status, Port Priority, Enable STP per Port

EDS-408A/405A Series User’s Manual					Featured Functions
	Enable STP per Port
		Setting	Description		Factory Default
		Enable/Disable	Select to enable the port as a node		Disabled
			on the Spanning Tree topology.
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	Increase this port’s priority as a
			node on the Spanning Tree	128
		selected by user	topology by entering a lower
			number.
	Port Cost
		Setting	Description		Factory Default
		Numerical value	Input a higher cost to indicate that
			this port is less suitable as a node	200000
		input by user
			for the Spanning Tree topology.

Port Status

This indicates the current Spanning Tree status of this port. The status values are Forwarding for normal transmission, and Blocking to block transmission.

Configuration Limits of RSTP/STP

The Spanning Tree Algorithm places limits on three of the configuration items previously described:

[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 EtherDevice Switch’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 a multitude of ways. One solution is simply to increase the Forwarding Delay to at least 11 sec.

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