Allied Telesis AT-S41 - Manual (Page 82)

AT-S41 User’s Guide
82
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 for the ports on the AT-8350GB Fast Ethernet switch are
adjustable through the management software. Below are the default
port cost 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 is a value that you can adjust on a per port
basis on the switch.
Handling
Topology
Changes
If there is a change in the network topology due to a failure, removal, or
addition of any active components, the active topology also changes.
This may trigger a change in the state of some blocked ports. However, a
change in a port state is not activated immediately.
Table 1 AT-8350GB Port Costs
Port Speed Port Cost
10 Mbps 10
100 Mbps 10
1000 Mbps 4