3Com Switch 4800G 24-Port - Page 226

226 CHAPTER 23: 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:
c
CAUTION:
■ 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.
■ When the path cost of a port is changed, MSTP will re-calculate 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.
Configuring Port Priority The priority of a port is an import basis that determines whether the port can be
elected as the root port of device. If all other conditions are the same, the port
with the highest priority will be elected as the root port.
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 43 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
Required
Use either command.
Configurations made in
Ethernet interface view will
take effect on the current
port only; configurations
made in port group view
will take effect 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.