Allied Telesis AT-S60 - Manual (Page 230)

Chapter 15: STP and RSTP
Section II: Advanced Features 230
Note
Spanning tree is disabled by default on the switch.
Note
For information about Multiple Spanning Tree, see Chapter 16,
Multiple Spanning Tree Protocol (MSTP) on page 257.
Note
An AT-8411 TX line card with more than four ports functioning as
redundant links to other network devices can significantly retard the
speed of convergence for STP and RSTP. You can avoid this problem
by selecting ports on different line cards to function as redundant
links.
Bridge Priority
and the Root
Bridge
The first task that bridges perform when a spanning tree protocol is
activated on a network is the selection of a root bridge. A root bridge
distributes network topology information to the other network bridges
and is used by the other bridges to determine if there are redundant
paths in the network.
A root bridge is selected by a combination of a bridge priority number,
also referred to as the bridge identifier, and sometimes the bridge’s MAC
address. The bridge with the lowest bridge priority number in the
network is selected as the root bridge. If two or more bridges have the
same bridge priority number, of those bridges the one with the lowest
MAC address is designated as the root bridge.
The bridge priority number can be changed on an AT-8400 Series switch.
You can designate a switch on your network as the root bridge by giving
it the lowest bridge priority number. You might also consider which
bridge should function as the backup root bridge in the event you need
to take the primary root bridge off-line, and assign that bridge the
second lowest bridge identifier number.
The range for STP and RSTP bridge priority is 0 to 61,440 in increments of
4,096. The range is divided into sixteen increments. You set the
parameter by specifying the increment that represents the desired
bridge priority value. Table 9 on page 231 lists the bridge priority value
increments. For example, to set a bridge priority value on a switch to
45056, select increment 11. The default value is 32,768, increment 8.