EXP Computer S200 manual Other, Considerations

Spanning Tree Protocol Entity (STPE)

Other	One important consideration is based on the fact that any bridge downstream from
Considerations	the root bridge copies the message received on the root link (which is also passed
	along designated links), and the retained copy is constantly aged. If the age of the
	message reaches the value of Max Age, the bridge discards the stored message and
	chooses another link as the root link. Potentially this could result in a different bridge
	selected as the root bridge and in turn cause it to recalculate the root, root link, and
	designated links (recalculate the spanning tree). Since the root port generates the
	update message every Hello Time period (in seconds), it is obvious that the
	parameter Max Age should not have a value less than or in fact near the value of
	Hello Time. The spanning tree protocol (IEEE 802.1D) dictates that a bridge should
	enforce the following relationship:
	Max Age >= 2 x (Hello Time - 1)
	In the PathBuilder S200 series switch this rule is not strictly enforced by CTP
	configuration checks. You should check that the values are satisfactory for the
	operational environment. The Max Age range of values is 6 to 40 and the default is
	20. The Hello Time range of values is 1 to 4 and the default is 2 seconds.
	These values allow the enforcement of timer relationships for any reasonable choice
	of values. The factor of two between Max Age and Hello Time allows one of the
	hello messages to be lost due to, for example, congestion.
	In general, increasing the value of Max Age lessens the chance of a false timeout due
	to a delay of the hello message. When bridge traffic must compete with other traffic
	on WAN links, setting this value can become an important consideration. On the
	other hand, beyond a certain point, a large value for Max Age may cause the
	detection of a true fault to be prolonged beyond what is desired. The Hello Time
	should be considered similarly:
	• Too low a value causes frequent transmission of the message, resulting in
	network overhead.
	• Too long an interval between transmissions forces a longer Max Age which
	results in lack of responsiveness to failure situations.
	In spite of the overhead, a short Hello Time helps in cases where the message might
	be inadvertently lost in the network (not likely) or where a short convergence time
	for the spanning tree is desired.

96 Bridging