3Com Switch 7757, Configuration Manual, Page: 253 / 940

Root Bridge Configuration 253
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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.
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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is obsolete.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:
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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.
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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.
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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.
Table 179 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).