BigIron RX Series Configuration Guide 1215
53-1002253-01
Configuring OSPFv3
48
Changing the reference bandwidth for the cost on OSPFv3
interfaces
Each interface on which OSPFv3 is enabled has a cost associated with it. The Brocade device
advertises its interfaces and their costs to OSPFv3 neighbors. For example, if an interface has an
OSPF cost of ten, the Brocade device advertises the interface with a cost of ten to other OSPF
routers.
By default, an interface’s OSPF cost is based on the port speed of the interface. The software uses
the following formula to calculate the cost.
Cost = reference-bandwidth/interface-speed
By default, the reference bandwidth is 100 Mbps. If the resulting cost is less than 1, the software
rounds 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 1
The 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 VLAN
that contains the virtual interface.
You can change the default reference bandwidth from 100 Mbps to a value from 1 – 4294967
Mbps.
If a change to the reference bandwidth results in a cost change to an interface, the Brocade device
sends a link state update to update the costs of interfaces advertised by the Brocade device.
If you specify the cost for an individual interface, the cost you specify overrides the cost calculated
by the software.
Some interface types are not affected by the reference bandwidth and always have the same cost
regardless 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 not
affected 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 500
The 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
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