182 CHAPTER 13: MSTP CONFIGURATION
Normally, the path cost of a port in full-duplex mode is slightly less than that of the port in half-duplex mode.When calculating the path cost of an aggregate link, the 802.1D-1998 standard does not take the number of the aggregated links into account, whereas the 802.1T standard does so by using the following equation:Path cost = 200,000,000/link transmission speedWhere, the link transmission speed is the sum of the speeds of the unblocked ports for the aggregate link measured in 100 kbps units.Configuring the path cost of a portChanges of path costs can cause MSTP to redetermine the roles of ports, resulting in state transition of ports. If you provide 0 for the instance-id argument when executing the stp cost command, the command sets the path cost of the CIST.
100 Mbps Half-Duplex
Full-Duplex
Aggregated Link 2 Ports
Aggregated Link 3 Ports
Aggregated Link 4 Ports
19
18
15
15
15
200,000
199,999
100,000
66,666
50,000
200
200
180
160
140
1,000 Mbps Full-Duplex
Aggregated Link 2 Ports
Aggregated Link 3 Ports
Aggregated Link 4 Ports
4
3
3
3
20,000
10,000
6,666
5,000
20
18
16
14
10 Gbps Full-Duplex
Aggregated Link 2 Ports
Aggregated Link 3 Ports
Aggregated Link 4 Ports
2
1
1
1
2,000
1,000
666
500
2
1
1
1

Table162 Configure the path costs of specified ports in system view

Operation Command Description
Enter system view system-view -
Configure the path costs of
specified ports
stp interface interface-list [
instance instance-id ] cost
cost
Required
By default, an MSTP-enabled
switch calculates path costs of its
ports automatically.

Table163 Configure the path cost of a port in Ethernet port view

Operation Command Description
Enter system view system-view -
Enter Ethernet port view interface interface-type
interface-number
-
Configure the path cost of the
port
stp [ instance instance-id ]
cost cost
Required
By default, an MSTP-enabled
switch calculates path costs of its
ports automatically.

Table161 Transmission speeds and the corresponding path costs (continued)

Transmission
speed
Operation mode
(half-/full-duplex) 802.1D-1998 IEEE 802.1t
3Com-3Com
standard