Allied Telesis AT-S62 - Manual (Page 485)

AT-S62 Management Software Menus Interface User’s Guide
Section IV: Spanning Tree Protocols 485
Bridge Priority
and the Root
Bridge
The first task that bridges running spanning tree perform 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 the 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.
By changing the bridge priority number in the AT-S62 software, 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 offline, and assign that bridge the second
lowest bridge identifier number.
The bridge priority has a range 0 to 61440 in increments of 4096. There
are sixteen increments. You specify the increment that represents the
desired bridge priority value. The increments are shown in Table 11.
Path Costs and Port Costs
After the root bridge has been selected, the bridges must determine if the
network contains redundant paths. If one is found, they must select a
preferred path while placing the redundant paths in a backup or blocking
state.
Where there is only one path between a bridge and the root bridge, the
bridge is referred to as the designated bridge and the port through which
Table 11. 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