71Configuring path costs of portsPath cost is a parameter related to the rate of a port. On a spanning tree device, a port can have differentpath costs in different MSTIs. Setting appropriate path costs allows VLAN traffic flows to be forwardedalong different physical links, achieving VLAN-based load balancing.You can have the device automatically calculate the default path cost, or you can configure the path costfor ports.Specifying a standard for the device to use when it calculates the default path costYou can specify a standard for the device to use in automatic calculation for the default path cost. Thedevice supports the following standards:• dot1d-1998—The device calculates the default path cost for ports based on IEEE 802.1d-1998.• dot1t—The device calculates the default path cost for ports based on IEEE 802.1t.• legacy—The device calculates the default path cost for ports based on a private standard.Follow these steps to specify a standard for the device to use when it calculates the default path cost:To do… Use the command… RemarksEnter system view system-view —Specify a standard for the device touse when it calculates the defaultpath costs of its portsstp pathcost-standard{ dot1d-1998 | dot1t | legacy }Requiredlegacy by default.CAUTION:If you change the standard that the device uses to calculate the default path costs, you restore the pathcosts to the default.NOTE:• When it calculates path cost for an aggregate interface, IEEE 802.1t takes into account the number ofSelected ports in its aggregation group, but IEEE 802.1d-1998 does not. The calculation formula of IEEE802.1t is: Path cost = 200,000,000/link speed (in 100 kbps), where link speed is the sum of the linkspeed values of the Selected ports in the aggregation group.• When the link speed of a single port exceeds 10 Gbps and the standard for default path cost calculationis dot1d-1998 or legacy, the path cost of a single port or an aggregate interface takes the smallestvalue. This affects the reasonableness of forwarding path selection. To solve this problem, use dot1t asthe standard for default path cost calculation, or manually set the path cost for a port (Configuring pathcosts of ports).Table 13 Mappings between the link speed and the path costLink speed Port typePath costIEEE802.1d-1998 IEEE 802.1t Private standard0 N/A 65535 200,000,000 200,00010 MbpsSingle port1002,000,000 2000Aggregate interfacecontaining 2 Selected ports 1,000,000 1800
