3Com 5500-SI Configuration Manual page 174

174 C 13: MSTP C
HAPTER ONFIGURATION
To solve this problem, MSTP adopts the state transition mechanism. With this
mechanism, new root ports and designated ports must go through an intermediate
state to the forwarding state, so that the new BPDUs can be advertised throughout
the network. The introduced delay is dictated by the Forward delay argument.
Hello time: Indicates the interval in which the switch checks the connectivity of
■
links.
A switch sends Hello packets to its neighboring switches in the specified Hello time
interval to check the connectivity of links.
Max age: Indicates the timeout time of BPDUs. Obsolete BPDUs will be discarded.
■
Configuration procedure
Table 149 Configure MSTP time parameters
Operation
Enter system view
Configure the Forward delay
Configure the Hello time
Configure the Max age
All switches in a switched network adopt the settings of the three time parameters
configured on the CIST root bridge.
The value of the Forward delay argument depends on the network diameter.
Normally, a greater network diameter requires a greater Forward delay. A too small
Forward delay may result in temporary redundant paths. And a too great Forward
delay may cause a network unable to resume connectivity in a relatively long time.
The default is recommended.
An adequate Hello time enables a switch to detect link problems in time without
consuming too many network resources. A too great Hello time may cause normal
links to be regarded as failed when only some packets get lost, which in turn causes
spanning trees to be recalculated. And a too small Hello time causes duplicated
BPDUs to be sent frequently, which increases the load of the switches and wastes
network resources. The default is recommended.
As for the Max age argument, if it is too small, network congestions may be falsely
regarded as link problems, which causes the spanning trees to be frequently
regenerated. If it is too large, link problems may not be found in time, which causes
spanning trees unable to be regenerated in time, making the network less adaptive.
The default is recommended.
The settings of the three MSTP time parameters must satisfy the following expressions
to prevent frequent network jitters:
2 * (Forward delay 1 second) >= Max age
Max age >= 2 * (Hello time + 1 second)
Command
system-view
stp timer forward-delay
centi-seconds
stp timer hello centi-seconds Required
stp timer max-age
centi-seconds
Description
—
Required
The Forward delay defaults to
1,500 centiseconds (15 seconds).
The Hello time defaults to 200
centiseconds (2 seconds).
Required
The Max age defaults to 2,000
centiseconds (20 seconds).