3Com Switch 8800 Advanced Software V5 Configuration Guide

122 CHAPTER 11: MSTP CONFIGURATION
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In the calculation of the path cost value of an aggregated link, 802.1D-1998 does
not take into account the number of ports in the aggregated link. Whereas,
802.1T takes the number of ports in the aggregated link into account. The
calculation formula is: Path Cost = 200,000,000/link speed (in 100 kbps), where
link speed is the sum of the link speed values of the non-blocked ports in the
aggregated link.
Configuring Path Costs of Ports
Follow these steps to configure the path cost of ports:
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■ When the path cost of a port is changed, MSTP will re-compute the role of the
port and initiate a state transition. If you use 0 as instance-id, you are setting
the path cost of the CIST.
■ If you change the standard that the device uses in calculating the default path
cost, the port path cost value set through the stp cost command will be out of
effect.
1000 Mbps Single Port
Aggregated Link
2 Ports
Aggregated Link
3 Ports
Aggregated Link
4 Ports
4
4
4
4
20,000
10,000
6,666
5,000
20
18
16
14
10 Gbps Single Port
Aggregated Link
2 Ports
Aggregated Link
3 Ports
Aggregated Link
4 Ports
2
2
2
2
2,000
1,000
666
500
2
1
1
1
Table 14 Link speed vs. path cost
Link speed Duplex state 802.1D-1998 802.1t
Private
standard
To do... Use the command... Remarks
Enter system view system-view -
Enter Ethernet
interface view
or port group
view
Enter Ethernet
interface view
interface interface-type
interface-number
User either command
Configured in Ethernet
interface view, the setting is
effective on the current port
only; configured in port
group view, the setting is
effective on all ports in the
port group
Enter port
group view
port-group { manual
port-group-name |
aggregation agg-id }
Configure the path cost of the
port(s)
stp [ instance instance-id ]
cost cost
Required
By default, MSTP
automatically calculates the
path cost of each port