H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 180)

20-22
These three timers set on the root bridge of the CIST apply on all the devices on the entire switched
network.
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 resume connectivity. 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 on a link for 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
calculation and may take network congestion to 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 calculation,
thus reducing the auto-sensing capability of the network. We recommend that you use the default
setting.
The setting 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) ƒ max age
z
Max age ƒ 2 × (hello time + 1 second)
We recommend that you specify the network diameter in the stp root primary command and let MSTP
automatically calculate an optimal setting 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