3Com PathBuilder S200 Series Owner's Manual page 100

Spanning Tree Protocol Entity (STPE)
Other
Considerations
96
One important consideration is based on the fact that any bridge downstream from
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.
Bridging