Allied Telesis AT-S25 - Manual (Page 95)

AT-S25 Management Software User’s Guide
95
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-8316F or an AT-8324 Switch are
adjustable through the management software. For either STP or RSTP,
the port costs have a range of from 0 to 20 000 000. This range allows
you to have more control over path costs.
These port costs also feature an Auto-Detect feature. This features allows
either STP or RSTP 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 on the ports when the switch is operating in
either STP or RSTP. Table 7 lists the ports cost with Auto-Detect.
You could override Auto-Detect and set the port cost manually.
Table 7 Auto-Detect Port Costs
Port Speed Port Cost
10 Mbps 2 000 000
100 Mbps 200 000
1000 Mbps 20 000