BigIron RX Series Configuration Guide 119553-1001986-01Configuring OSPFv3 48Changing the reference bandwidth for the cost on OSPFv3interfacesEach interface on which OSPFv3 is enabled has a cost associated with it. The Brocade deviceadvertises its interfaces and their costs to OSPFv3 neighbors. For example, if an interface has anOSPF cost of ten, the Brocade device advertises the interface with a cost of ten to other OSPFrouters.By default, an interface’s OSPF cost is based on the port speed of the interface. The software usesthe following formula to calculate the cost.Cost = reference-bandwidth/interface-speedBy default, the reference bandwidth is 100 Mbps. If the resulting cost is less than 1, the softwarerounds the cost up to 1. The default reference bandwidth results in the following costs:• 10 Mbps port’s cost = 100/10 = 10• 100 Mbps port’s cost = 100/100 = 1• 1000 Mbps port’s cost = 100/1000 = 0.10, which is rounded up to 1• 155 Mbps port’s cost = 100/155 = 0.65, which is rounded up to 1• 622 Mbps port’s cost = 100/622 = 0.16, which is rounded up to 1• 2488 Mbps port’s cost = 100/2488 = 0.04, which is rounded up to 1The bandwidth for interfaces that consist of more than one physical port is calculated as follows:• Trunk group – The combined bandwidth of all the ports.• Virtual (Ethernet) interface – The combined bandwidth of all the ports in the port-based VLANthat contains the virtual interface.You can change the default reference bandwidth from 100 Mbps to a value from 1 – 4294967Mbps.If a change to the reference bandwidth results in a cost change to an interface, the Brocade devicesends a link state update to update the costs of interfaces advertised by the Brocade device.NOTEIf you specify the cost for an individual interface, the cost you specify overrides the cost calculatedby the software.Some interface types are not affected by the reference bandwidth and always have the same costregardless of the reference bandwidth in use:• The cost of a loopback interface is always 0.• The cost of a virtual link is calculated using the Shortest Path First (SPF) algorithm and is notaffected by the auto-cost feature.• The bandwidth for tunnel interfaces is 9 Kbps and is not affected by the auto-cost feature.For example, to change the reference bandwidth to 500, enter the following command.BigIron RX(config-ospf6-router)# auto-cost reference-bandwidth 500The reference bandwidth specified in this example results in the following costs:• 10 Mbps port’s cost = 500/10 = 50• 100 Mbps port’s cost = 500/100 = 5
