3Com 400 Family, Configuration Manual

Page: 175 / 344

RSTP Configuration 173
After a Switch is configured as primary root bridge or secondary root bridge, you
cannot modify the bridge priority of the Switch.
A Switch can either be a primary or secondary root bridge, but not both of them.
If the primary root of a spanning tree instance is down or powered off, the
secondary root will take its place, unless you configure a new primary root. Of two
or more configured secondary root bridges, RSTP selects the one with the smallest
MAC address to take the place of the failed primary root.
To configure a Switch as the root of the spanning tree instance, you can specify its
priority as 0 or simply set it as the root, using the command.
It is not necessary to specify two or more roots for an STI — do not specify the
root for an STI on two or more Switches.
You can configure more than one secondary root for a spanning tree through
specifying the secondary STI root on two or more Switches.
Generally, 3Com recommends designating one primary root and two or more
secondary roots for a spanning tree.
By default, a Switch is neither the primary root nor the secondary root of the
spanning tree.
Set Forward Delay of a
Specified Bridge
Link failure will cause recalculation of the spanning tree and change its structure.
However, the newly calculated configuration BPDU cannot be propagated
throughout the network immediately. If the newly selected root port and
designated port begin to forward data frames right away, this can cause an
occasional loop. Accordingly, the protocol adopts a state transition mechanism,
that is, the root port and the designated port must undergo a transition state for a
period of Forward Delay before they transition to the forwarding state and resume
data frame forwarding. This delay ensures that the new configuration BPDU has
been propagated throughout the network before the data frame forwarding is
resumed.
You can use the following command to set the Forward Delay for a specified
bridge.
Perform the following configurations in System View.
Table 176 Set Forward Delay of a Specified Bridge
Forward Delay of the bridge is related to the diameter of the Switching network.
As a rule, the larger the network diameter, the longer the Forward Delay. Note
Disqualify the current Switch as the primary or secondary root.
undo stp root
Operation Command
Operation Command
Set Forward Delay of a specified bridge
stp timer forward-delay
centiseconds
Restore the default Forward Delay of
specified bridge
undo stp timer forward-delay