(Optional) Configuring a Path Cost of a Port in an MSTI

The Multiple Spanning Tree Protocol (MSTP) path cost determines root port selection in a Multiple Spanning Tree Instance (MSTI). The port with the lowest path cost to the root bridge is selected as a root port.

Context

A path cost is port-specific, which is used by MSTP as a reference to select a link.

Path costs of a port are an important basis for calculating spanning trees. If you set different path costs for a port in different Multiple Spanning Tree Instances (MSTIs), you can make VLAN traffic be transmitted along different physical links and thus carry out VLAN load balancing.

Use the Huawei private calculation method as an example. The following table maps link rates and default path cost values of ports.

**Table 1** Mapping between link rates and path cost values
Link Rate	Recommended Path Cost	Recommended Path Cost Range	Path Cost Range
10 Mbit/s	2000	200 to 20000	1 to 200000
100 Mbit/s	200	20 to 2000	1 to 200000
1 Gbit/s	20	2 to 200	1 to 200000
10 Gbit/s	2	2 to 20	1 to 200000
Higher than 10 Gbit/s	1	1 to 2	1 to 200000

On a network where loops occur, you are recommended to set a relatively large path cost for the port at a low link rate. MSTP puts the port with the large path cost in the Blocking state and blocks the link where this port resides.

Procedure

Run system-view
The system view is displayed.

Run stp pathcost-standard { dot1d-1998 | dot1t | legacy }

A path cost calculation method is configured.

All devices on a network must use the same path cost calculation method.

Table2 Path cost list lists path costs defined by the IEEE 802.1D-1998 standard, IEEE 802.1T standard, and Huawei legacy standard. Different vendors use different standards.

**Table 2** Path cost list
Interface Rate	Interface Mode	Recommended STP Path Cost
Interface Rate	Interface Mode	IEEE 802.1D-1998 Standard	IEEE 802.1T Standard	Huawei Legacy Standard
0	-	65535	200,000,000	200,000
10 Mbps	Half-Duplex	100	2,000,000	2000
	Full-Duplex	99	1,999,999	1999
	Aggregated Link 2 Ports	95	1,000,000	1800
	Aggregated Link 3 Ports	95	666,666	1600
	Aggregated Link 4 Ports	95	500,000	1400
100 Mbps	Half-Duplex	19	200,000	200
	Full-Duplex	18	199,999	199
	Aggregated Link 2 Ports	15	100,000	180
	Aggregated Link 3 Ports	15	66,666	160
	Aggregated Link 4 Ports	15	50,000	140
1000 Mbps	Full-Duplex	4	20,000	20
	Aggregated Link 2 Ports	3	10,000	18
	Aggregated Link 3 Ports	3	6666	16
	Aggregated Link 4 Ports	3	5000	14
10 Gbps	Full-Duplex	2	2000	2
	Aggregated Link 2 Ports	1	1000	1
	Aggregated Link 3 Ports	1	666	1
	Aggregated Link 4 Ports	1	500	1

Run interface interface-type interface-number
The view of the Ethernet interface participating in STP calculation is displayed.

The following configuration can be configured both on a Layer 2 interface and a Layer 3 interface.
Run stp instance instance-id cost cost
A path cost is set for the port in the current MSTI.
- When the Huawei proprietary calculation method is used, cost ranges from 1 to 200000.
- When the IEEE 802.1d standard method is used, cost ranges from 1 to 65535.
- When the IEEE 802.1t standard method is used, cost ranges from 1 to 2000,000,00.
Run commit
The configuration is committed.