3Com 8807 Configuration Manual page 142

142 C 17: MSTP R
HAPTER
STP Path Cost
Calculation Standards on
STP port
-
EGION CONFIGURATION
Table 120 Configure the path cost of a port
Operation
Configure the path cost of a port
Restore the default path cost of a port
You can configure the path cost of a port with either of the earlier-mentioned
measures. Upon the change of path cost of a port, MSTP will recalculate the port
role and transit the state. When instance-id takes 0, it indicates to set the path
cost on the CIST.
By default, MSTP is responsible for calculating the path cost of a port.
The 3Com Switch 8800 Family Series Routing Switches support 3Com's legacy
path cost calculation. DOT1T calculation and DOT1D-1998 calculation can also be
used. By default, legacy standard is applied for Switch 8800 Family series.
The port rate must be obtained first before calculating the path cost of a port as
the path cost is associated with the port rate. The three standards use their own
way to work out the port rate, based on which each standard calculates the path
cost of the by certain algorithm.
DOT1T calculation standard
1 Calculating the rate
Aggregation port
■
The rate of either a primary or a secondary port in an aggregation port group is
the sum of the port rates in the group. If a port is down, the rate is 0.
Non-aggregation port
■
The actual rate counts.
2 Calculating the path cost
Full-duplex and non-aggregation port at a rate less than 1 GE
■
Path cost = [200,000,000 / (rate × 10)] - 1
Other ports
■
Path cost = 200,000,000 / (rate × 10)
DOT1D-1998 calculation standard
1 Calculating the rate
Aggregation port
■
If the port is up, the actual rate counts. If the port is down, the rate is determined
by that of the port which goes up first in the aggregation group. If all the ports in
the aggregation group are down, the rate of the aggregation port is 0.
Non-aggregation port
■
Command
stp [ instance instance-id ] cost cost
undo stp [ instance instance-id ] cost