AT-S84 Management Software User’s Guide
Section II: Menus Interface 147
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. You can change the
bridge priority number for the switch.
The bridge priority has a range of 0X0000 to 0XF000 and is specified in
multiples of 0x1000.
After the convergence process has completed, there is only one path
between the switch and the root bridge. The active port on the switch
through which the bridge is communicating with the root bridge is called
the root port. Each switch in the spanning tree domain has a root port with
the exception of the root bridge, which has no root port.
Designated Bridge and Designated Port
The switch that is directly connected to the root port of the switch is called
the designated bridge. The port on the designated bridge that is connected
to the switch’s root port is called the designated port.
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.
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
port on a bridge is typically based on port speed. The faster the port, the
lower the port cost. The exception to this is the ports on the root bridge,
where all ports have a port cost of 0.
Path cost is the sum of the port costs between a bridge and the root
bridge.
Port cost also has an Auto-Detect feature. This feature allows spanning
tree to automatically set the port cost according to the speed of the port,
assigning a lower value for higher speeds. Auto-Detect is the default
setting.