H3C S7500 Series Operation Manual page 261

Operation Manual – MSTP
H3C S7500 Series Ethernet Switches
Caution:
The Forward delay parameter and the network diameter are correlated. Normally, a
large network diameter corresponds to a large Forward delay. A too small Forward
delay parameter may result in temporary redundant paths. And a too large Forward
delay parameter may cause a network unable to resume the normal state in time
after changes occurred to the network. The default value is recommended.
An adequate Hello time parameter enables a switch to be aware of link failures in
time without occupying too much network resources. Too large a Hello time
parameter may result in normal links being regarded as invalid when packets get
lost on them, which in turn results in spanning trees being regenerated. And too
small a Hello time parameter may result in duplicated configuration BPDUs being
sent frequently, which increases the work load of the switches and wastes network
resources. The default value is recommended.
As for the Max age parameter, if it is too small, network congestions may be falsely
regarded as link failures, which results in spanning trees being frequently
regenerated. If it is too large, link failures may be unable to be detected in time,
which in turn disables spanning trees being regenerated in time and makes the
network less adaptive. The default value is recommended.
As for the configuration of these three time-related parameters (that is, the Hello time,
Forward delay, and Max age parameters), the following formulas must be met to
prevent network jitter.
2 x (Forward delay – 1 second) >= Max age
Max age >= 2 x (Hello time + 1 second)
You are recommended to specify the network diameter of the switched network and the
Hello time by using the stp root primary or stp root secondary command. After that,
the three proper time-related parameters are determined automatically.
II. Configuration example
# Configure the Forward delay parameter to be 1,600 centiseconds, the Hello time
parameter to be 300 centiseconds, and the Max age parameter to be 2,100
centiseconds (assuming that the current switch operates as the CIST root bridge).
<H3C> system-view
[H3C] stp timer forward-delay 1600
[H3C] stp timer hello 300
[H3C] stp timer max-age 2100
1-17
Chapter 1 MSTP Configuration