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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 speed
Where, 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 port
Changes 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
Table 162 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.
Table 163 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.
Table 161 Transmission speeds and the corresponding path costs (continued)
Transmission
speed
Operation mode
(half-/full-duplex)
802.1D-1998 IEEE 802.1t
3Com-3Com
standard