3Com PathBuilder S200 Series, Owner'S Manual

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Bridging 99
T0008-16F Release 5.2M
Spanning Tree Protocol Entity (STPE)
Aging Timer
The Aging Timer is a configurable parameter found in the Bridge Parameters
Record. It allows learned station addresses to be aged in the station address cache
and deleted once their age has reached the value of the Aging Timer parameter. This
allows automatic updates for certain dynamic conditions, such as when a station is
physically moved from one part of the network to another. Provided the Aging Timer
is low enough to age out the station that is being moved, the entry is deleted and,
once the station becomes active at its new location, the bridge relearns its new
location and forwards packets to it properly.
The Aging Timer is also used when there is a topological change to the network.
Generally, if a bridge, based on the automatic spanning tree algorithm, notices that a
link must be moved to or from a block state (moved to or from the spanning tree),
then the bridge informs the root bridge by means of a special message sent out the
root port. This message includes a flag that indicates a topological change has
occurred. The root bridge, in turn, informs all the bridges in the network that a
change has occurred by setting a flag in the hello message. Once a bridge is informed
of a change in the topology by the hello flag, it switches the timer value used in aging
to a value equal to the Forward Delay time. This allows the bridge to more quickly
relearn new station locations, if a topology change occurs due to node or link
failures. Note that the topological change messages are sent at the same rate as the
Hello Time interval. This limits the speed and gives the lower time boundary so that
bridges can adjust to the change.