ZyXEL Communications MES-2110 manual Allowed range is 4 to 30 seconds, As a general rule

Chapter 11 Bridge

Table 26 Configuration > Bridge Menu > RSTP System Config

LABEL	DESCRIPTION
Forward Delay (sec)	This is the maximum time (in seconds) the MES-2110 will wait
	before changing states. This delay is required because every
	switch must receive information about topology changes before it
	starts to forward frames. In addition, each port needs time to
	listen for conflicting information that would make it return to a
	blocking state; otherwise, temporary data loops might result. The
	allowed range is 4 to 30 seconds.
	As a general rule:

	Note: 2 * (Forward Delay - 1) >= Max Age >= 2 * (Hello Time
	+ 1)
Max age (sec)	This is the maximum time (in seconds) the MES-2110 can wait
	without receiving a BPDU before attempting to reconfigure. All
	MES-2110 ports (except for designated ports) should receive
	BPDUs at regular intervals. Any port that ages out STP
	information (provided in the last BPDU) becomes the designated
	port for the attached LAN. If it is a root port, a new root port is
	selected from among the MES-2110 ports attached to the
	network. The allowed range is 6 to 40 seconds.
Undo	Click this to restore your last saved settings.
Refresh	Click this to reset the data for the field(s).
Apply	Click Apply to save your changes to the MES-2110’s run-time
	memory. The MES-2110 loses these changes if it is turned off or
	loses power, so use the Save Settings link in the navigation
	panel to save your changes to the non-volatile memory when you
	are done configuring.

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MES-2110 User’s Guide