Allied Telesis AT-S63 - Manual (Page 477)

AT-S63 Management Software Menus Interface User’s Guide
Section IV: Spanning Tree Protocols 477
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 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.
You can change the bridge priority number in the AT-S63 management
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. To make
this easier for you, the AT-S63 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 Table 9.
Path Costs and Port Costs
After the root bridge has been selected, the bridges must determine if the
network contains redundant paths and, if one is found, they must select a
preferred path while placing the redundant paths in a backup or blocking
state.
Table 9. 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