H3C S5810 Series, Руководство по эксплуатации

Страница: 124 / 820

1-23
z
The length of the forward delay time is related to the network diameter of the switched network.
Typically, the larger the network diameter is, the longer the forward delay time should be. Note that
if the forward delay setting is too small, temporary redundant paths may be introduced; if the
forward delay setting is too big, it may take a long time for the network to converge. We recommend
that you use the default setting.
z
An appropriate hello time setting enables the device to timely detect link failures on the network
without using excessive network resources. If the hello time is set too long, the device will take
packet loss as a link failure and trigger a new spanning tree calculation process; if the hello time is
set too short, the device will send repeated configuration BPDUs frequently, which adds to the
device burden and causes waste of network resources. We recommend that you use the default
setting.
z
If the max age time setting is too small, the network devices will frequently launch spanning tree
calculations and may take network congestion as a link failure; if the max age setting is too large,
the network may fail to timely detect link failures and fail to timely launch spanning tree calculations,
thus reducing the auto-sensing capability of the network. We recommend that you use the default
setting.
The settings of hello time, forward delay and max age must meet the following formulae; otherwise
network instability will frequently occur.
z
2 × (forward delay – 1 second) ú max age
z
Max age
ú
2 × (hello time + 1 second)
We recommend that you specify the network diameter with the
stp root primary
command and let
MSTP automatically calculate optimal settings of these three timers.
Configuration example
# Set the forward delay to 1,600 centiseconds, hello time to 300 centiseconds, and max age to 2,100
centiseconds.
<Sysname> system-view
[Sysname] stp timer forward-delay 1600
[Sysname] stp timer hello 300
[Sysname] stp timer max-age 2100
Configuring the Timeout Factor
After the network topology is stabilized, each non-root-bridge device forwards configuration BPDUs to
the downstream devices at the interval of hello time to check whether any link is faulty. Typically, if a
device does not receive a BPDU from the upstream device within nine times the hello time, it will
assume that the upstream device has failed and start a new spanning tree calculation process.
In a very stable network, this kind of spanning tree calculation may occur because the upstream device
is busy. In this case, you can avoid such unwanted spanning tree calculation by lengthening the timeout
time.