3Com Switch 8800 Family Configuration Guide

Configuring MSTP 139
Too short a Max Age may cause the network device frequently calculate the
spanning tree 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
recalculate the spanning tree in time, which will weaken the auto-adaptation
capacity of the network. The default value is recommended.
To avoid frequent network flapping, the values of Hello Time, Forward Delay and
Max Age should guarantee the following formulas.
2 x (Forward-Delay - 1 second) >= Max Age
Max Age >= 2 x (Hello Time + 1 second)
You are recommended to use the stp bridge-diameter command to specify the
network diameter and Hello Time of the switching network, and then MSTP will
automatically calculate and give the rather desirable values.
By default, Forward Delay is 15 seconds, Hello Time is 2 seconds, and Max Age is
20 seconds.
Setting the Timeout
Factor of a Specific
Bridge
A switch transmits hello packet regularly to the adjacent bridges to check if there
is link failure. Generally, if the switch does not receive the STP packets from the
upstream switch for 3 times of hello time, the switch will decide the upstream
switch is dead and will recalculate the topology of the network. Then, in a steady
network, the recalculation may be caused when the upstream is busy. In this case,
user can redefine the timeout interval to a longer time to avoid this kind of
meaningless recalculation.
You can use the following command to set the multiple value of hello time of a
specified bridge.
Perform the following configurations in system view.
It is recommended to set 5, 6 or 7 as the timeout factor in the steady network.
By default, the timeout factor of the switch is 3.
Configuring the Max
Transmission Speed on a
Port
The max transmission speed on a port specifies how many MSTP packets will be
transmitted via the port every Hello Time.
The max transmission speed on a port is limited by the physical state of the port
and the network structure. You can configure it according to the network
conditions.
You can configure the max transmission speed on a port in the following ways.
Tab le 114 Setting the timeout factor of a specific switch
Operation Command
Set the timeout factor of a specified switch stp timer-factor number
Restore the default timeout factor undo stp timer-factor