3Com 4007 Implementation Manual page 193

If the functioning forward delay is on the high end of the allowable
range (4-30 seconds, 15 is the default), this allows more time for an
STP network to stabilize but it also delays the transition of ports to the
forwarding state because ports are held longer in each of the listening
and learning states. However, when this value is used as the aging
interval, the interval will probably still be much lower than the normal
aging interval (range 10 - 1,000,000 seconds; 300 is the default).
If the functioning forward delay is on the low end of the range and
STP is enabled, ports may transition to the forwarding state before the
network topology stabilizes, thereby potentially causing loops.
If you select the
agingOnly
3Com recommends that you set the bridge spanningTree
stpForwardDelay value to the lowest possible value — 4 seconds. This
will ensure that the address flushing is performed as quickly as
possible.
If you select either
state on Layer 2 Switching Modules, the reporting of link up and
down events is delayed by the value of the functioning forward delay.
This delay helps ensure that the application Transcend Enterprise VLAN
Manager (EVM) operates correctly.
EVM associates a MAC address with each port that it assigns to a
VLAN. If MAC addresses are not flushed out fast enough after a link
up or link down event, the next EVM query may detect an old address
on a given port and use it to determine VLAN membership for the
port. The delay in sending traps allows sufficient time for addresses to
be aged out before EVM queries arrive to retrieve the MAC address on
a given port, thereby guaranteeing correct VLAN assignments.
The delay is not necessary on Multilayer Switching Modules because,
irrespective of the STP state, the modules immediately flush dynamic
addresses associated with down or disabled ports.
The "Aging Only" option was created primarily for customers with
Layer 2 Switching Modules who use 3Com Transcend EVM but do not
want to enable STP. If you do not want to enable STP and if you do not
want to accelerated address aging on link down events and if you do
want traps reported immediately, then set the STP state to
option on Layer 2 Switching Modules,
or
enabled agingOnly
Address Aging
for the bridge-wide STP
disabled
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