Allied Telesis AT-S39 - Manual (Page 98)

Section II: Local and Telnet Management
98
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-8000 Series switch.
You can designate which switch on your network you want 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.
With STP, the bridge priority has a range of from 0 to 65535. You can
select any value within that range.
With RSTP, the range is slightly less, from 0 to 61440. Furthermore, you
can only select a value that is a multiple of 4096. To make this easier for
you, the management software divides the range into increments. You
specify the increment that represents the desired bridge priority value.
The range is divided into sixteen increments, as shown in the following
table.
Table 4 RSTP Bridge Priority Value Increments
Increment
Bridge
Priority
Increment
Bridge
Priority
0 0 8 32768
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