Allied Telesis AT-S39 - Manual (Page 84)

Section II: Local and Telnet Management
84
Finding and
Resolving
Redundant
Paths
Once 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 a 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
designated 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
evaluation of port 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.
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.
The port costs of the ports on the AT-8024 and AT-8024GB Fast Ethernet
Switches are adjustable through the management software. Below are
the default values.
The cost of a path is cumulative; the final cost of a path is the value of all
ports between a bridge and the root bridge.
If two paths have the same port cost, the preferred path is selected
through port priority. This too is a value that you can adjust on a per port
basis on the switch.
Table 1 Port Costs for AT-8024 and AT-8024GB Switches
Port Speed Port Cost
10 Mbps 100
100 Mbps 10
1000 Mbps 4