User's Manual

Chapter 12: Setting Switch Spanning Tree Protocols
140
 New Priority (0-15)— Assigns the switch a priority number. The
device that has the lowest priority number in the spanning tree
domain becomes the root bridge. If two or more devices have the
same priority value, the device with the numerically lowest MAC
address becomes the root bridge.
The range is 0 to 61,440, in increments of 4,096. The range is
divided into the sixteen increments listed in Table 2. You specify
the increment that represents the desired bridge priority value. The
default value is 32,768 (increment 8).
Note
Set the hello time, forward delay, and max-age fields according to
the following formulas, as specified in IEEE Standard 802.1d:
max-age <= 2 x (forward time - 1.0 second)
max-age => 2 x (hello time + 1.0 second)
 Hello Time— Indicates the frequency that the switch sends
spanning tree configuration information when it is the root bridge or
is trying to become the root bridge.
 Forward Delay— Indicates the forward time parameter on the
switch. This field specifies how long the ports remain in the
listening and learning states before they transition to the
forwarding state.
The Forward Delay value is active only when the switch is acting
as the root bridge of the spanning tree domain. Switches that are
not acting as the root bridge use a dynamic value supplied by the
root bridge.
Table 2. STP Bridge Priority Value Increments
Increment
Bridge
Priority
Increment
Bridge
Priority
00832768
1 4096 9 36864
2 8192 10 40960
3 12288 11 45056
4 16384 12 49152
5 20480 13 53248
6 24576 14 57344
7 28672 15 61440