Contemporary Control Systems Compact Managed Switches Software Manual for Web Browser - Manual (Page 44)

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4.4.9.2.2 STP Port Settings
The example shows the screen for an 8-port unit. The screen for a 16- or 24-port device
would extend the panel to include all appropriate ports. In this panel you can adjust :
Priority (of an individual port)
With the
Priority field, you can modify the priorities of individual ports to affect RSTP path
choices in the local vicinity of the switch. A lower value means a higher priority. Priority
settings differ from path costs which are cumulative in calculating a total path from
periphery to root. The Priority value only acts locally so you can force RSTP to favour a
certain path emerging from the switch in question when two paths from the switch are
otherwise equal. This field’s default value is 128 and its value can be toggled in 16 steps
from 0–240 where each increment has a value of 16.
Path Cost
In determining the most efficient path for conveying messages between the periphery of the
tree and its root, one of the factors RSTP relies on is “path costs”. A typical link operates at
either 10 or 100 Mbps and a port sending traffic to that link is assigned a “cost” derived from
the link data rate. The default for the Path Cost field is 19 (the nominal RSTP port cost for
a 100 Mbps link). Nominal values of port path costs and the suggested ranges through
which these values might vary in most networks are listed in Table 1. Path Cost can be
set from 1–65535. When should you set a port path cost to a very high value? Although
the associated link might operate at 10 Mbps, for example, the non-RSTP end of the link
might have a very slow device such as a dial-up modem that could slow traffic drastically.
In such a situation, you would likely want to raise this port path cost value to force RSTP to
only use this path as a last resort.
Table 1 — Port Path Costs
(Figure 35, fourth panel)