Allied Telesis AT-S88 - Manual (Page 105)

AT-S88 Management Software User’s Guide
97
You can change the bridge priority number in the AT-S88 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. To make this easier for you, the
AT-S88 management software divides the range into increments of 4096.
The valid bridge priority values that you can enter are shown in Table 4.
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.
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
the bridge is communicating with the root bridge is referred to as the root
port.
If redundant paths exist, the bridges that are a part of the paths must
determine which path will be the primary, active path, and which path(s)
will be placed in the standby, blocking mode. This is accomplished by an
determination of path costs. The path offering the lowest cost to the root
bridge becomes the primary path and all other redundant paths are placed
into blocking state.
Path cost is determined through an evaluation of port costs. Every port on
a bridge participating in STP has a cost associated with it. The cost of a
Table 4. Bridge Priority Value Increments
Bridge
Priority
Bridge
Priority
0 32768
4096 36864
8192 40960
12288 45056
16384 49152
20480 53248
24576 57344
28672 61440