3Com 4200G 12-Port Configuration Manual page 198

When calculating the path cost of an aggregated link, the 802.1D-1998 standard does not take the
number of the ports on the aggregated link into account, whereas the 802.1T standard does. The
following formula is used to calculate the path cost of an aggregated link:
Where, "link transmission rate" is the sum of the rates of all the unblocked ports on the aggregated link
measured in 100 Kbps.
Configure the path cost for specific ports
Follow these steps to configure the path cost for specified ports in system view:
To do...
Enter system view
Configure the path cost for
specified ports
Follow these steps to configure the path cost for a port in Ethernet port view:
To do...
Enter system view
Enter Ethernet port view
Configure the path cost for the
port
Changing the path cost of a port may change the role of the port and put it in state transition. Executing
the stp cost command with the instance-id argument being 0 sets the path cost on the CIST for the port.
Configuration example (A)
# Configure the path cost of GigabitEthernet 1/0/1 in MSTI 1 to be 2,000.
1) Perform this configuration in system view
<Sysname> system-view
[Sysname] stp interface GigabitEthernet 1/0/1 instance 1 cost 2000
2) Perform this configuration in Ethernet port view
<Sysname> system-view
[Sysname] interface GigabitEthernet 1/0/1
[Sysname-GigabitEthernet1/0/1] stp instance 1 cost 2000
Configuration example (B)
# Configure the path cost of GigabitEthernet 1/0/1 in MSTI 1 to be calculated by the MSTP-enabled
switch according to the IEEE 802.1D-1998 standard.
1) Perform this configuration in system view
<Sysname> system-view
[Sysname] undo stp interface GigabitEthernet 1/0/1 instance 1 cost
Path cost = 200,000,000 / link transmission rate
Use the command...
system-view
stp interface interface-list
[ instance instance-id ] cost
cost
Use the command...
system-view
interface interface-type
interface-number
stp [ instance instance-id ]
cost cost
1-31
Remarks
—
Required
An MSTP-enabled switch can
calculate path costs for all its
ports automatically.
Remarks
—
—
Required
An MSTP-enabled switch can
calculate path costs for all its
ports automatically.