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When calculating path cost for an aggregate interface, 802.1d-1998 does not take into account the
number of member ports in its aggregation group as 802.1t does. The calculation formula of 802.1t 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 aggregation group.
Configuring Path Costs of Ports
Follow these steps to configure the path cost of ports:
To do... Use the command... Remarks
Enter system view
system-view
—
Enter Ethernet
interface view, or
Layer 2 aggregate
interface view
interface interface-type
interface-number
Enter
interface
view or port
group view
Enter port group view
port-group manual
port-group-name
Required
Use either command.
Configurations made in
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.
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.
z 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 invalid.
z 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 important factor in determining whether the port can be elected as the root
port of a device. If all other conditions are the same, the port with the highest priority will be elected as
the root port.
On an MSTP-enabled device, a port can have different priorities in different MSTIs, and the same port
can play different roles in different MSTIs, so that data of different VLANs can be propagated along
different physical paths, thus implementing per-VLAN load balancing. You can set port priority values
based on the actual networking requirements.
Configuration procedure
Follow these steps to configure the priority of a port or a group of ports:
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