3Com Switch 7750 Configuration Guide Guide

Root Bridge Configuration 235
■ The Hello time parameter is for link testing.
A switch regularly sends hello packets to other switches in the interval specified by
the Hello time parameter to test the links.
■ The Max age parameter is used to judge whether or not a configuration BPDU
is obsolete. Obsolete configuration BPDUs will be discarded.
Configuration procedure
All switches in a switched network adopt the three time-related parameters
configured on the CIST root bridge.
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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 is
recommended.
■ An adequate Hello time parameter enables a switch to be aware of link
problems in time without occupying too much network resources. A too large
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 a too small 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 is recommended.
■ As for the Max age parameter, if it is too small, network congestions may be
falsely regarded as link problems, which results in spanning trees being
frequently regenerated. If it is too large, link problems may be unable to be
found in time, which in turn handicaps spanning trees being regenerated in
time and makes the network less adaptive. The default 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.
Table 162 Configure MSTP time-related parameters
Operation Command Description
Enter system view system-view -
Configure the Forward delay
parameter
stp timer forward-delay
centiseconds
Required
The Forward delay parameter
defaults to 1,500
centiseconds (15 seconds).
Configure the Hello time
parameter
stp timer hello centiseconds
Required
The Hello time parameter
defaults to 200 centiseconds
(2 seconds).
Configure the Max age
parameter
stp timer max-age
centiseconds
Required
The Max age parameter
defaults to 2,000
centiseconds (20 seconds).