3Com 7700 Configuration Manual page 238

230 C 8: STP O
HAPTER PERATION
Forward delay is the switch state transition mechanism. The spanning tree will be
recalculated upon link faults and its structure will change accordingly. However,
the configuration BPDU recalculated cannot be immediately propagated
throughout the network. Temporary loops can occur if the new root port and
designated port forward data right after being elected. Therefore, the protocol
adopts a state transition mechanism. It takes a forward delay interval for the root
port and designated port to transit from the learning state to forwarding state.
The forward delay guarantees a period of time during which the new
configuration BPDU can be propagated throughout the network.
The switch sends a hello packet periodically, at an interval specified by the hello
timer, to check if there is any link fault.
Max age specifies when the configuration BPDU expires. The switch will discard
the expired configuration BPDU.
You can use the following command to configure the time parameters for the
switch.
Perform the following configuration in system view.
Table 26 Configure the Time Parameters of a Switch
Operation
Configure Forward Delay on the switch.
Restore the default Forward Delay of the
switch.
Configure Hello Time on the switch.
Restore the default Hello Time on the switch.
Configure Max Age on the switch.
Restore the default Max Age on the switch.
Every switch on the switching network adopts the values of the time parameters
configured on the root switch of the CIST.
Caution: The forward delay configured on a switch depends on the switching
network diameter. Generally, the forward delay is supposed to be longer when the
network diameter is longer. Note that too short a forward delay can redistribute
some redundant routes temporarily, while too long a forward delay can prolong
the network connection resuming. The default value is recommended.
A suitable hello time ensures that the switch can detect the link fault on the
network but occupy moderate network resources. The default value is
recommended. If you set too long a hello time, when there is pa acket dropped
over a link, the switch may consider it as link fault and the network device will
recalculate the spanning tree accordingly. However, for too short a hello time, the
switch frequently sends configuration BPDU, which adds its burden and wastes
the network resources.
Too short a max age can cause the network device to calculate the spanning tree
frequently and mistake the congestion as a link fault. However, if the max age is
too long, the network device may not be able to discover the link fault and
Command
stp timer forward-delay centiseconds
undo stp timer forward-delay
stp timer hello centiseconds
undo stp timer hello
stp timer max-age centiseconds
undo stp timer max-age