Allied Telesis AT-S39 - Manual (Page 99)

AT-S39 User’s Guide
99
Path Costs and Port Costs
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 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 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 simply the cumulation of the port costs between a bridge
and the root bridge.
The port costs of the ports on an AT-8000 Series switch are adjustable
through the management software, but the range is different
depending on whether you are using STP or RSTP.
For STP, the range is 1 to 65535. You can assign a port a port cost of any
value within the range. Below are the default values.
In RSTP, the range is much greater: 0 to 20 000 000. This greater range
allows you to have more control over path costs.
Table 5 STP Default Port Costs
Port Speed Port Cost
10 Mbps 10
100 Mbps 10
1000 Mbps 4