IPv6: OSPF Cost
OSPF uses a reference bandwidth of 100 Mbps (10^8) for cost calculation. The formula to calculate the cost is reference bandwidth divided by interface bandwidth ; Interface Cost= 10^8/(interface bandwidth). Calculated values are rounded up to the nearest single decimal value. This means that OSPF will calculate interfaces with speeds higher than 10 Mbps as a cost of 1 (Table 1). T he result of this is that OSPF routers in the network cannot make an accurate path calculation when comparing fast Ethernet interface versus a gigabit Ethernet interface as both interfaces has a cost of 1.  Interface Type Cost 100 Gbps 1 40 Gbps 1 10 Gbps 1 10 Gbps 1 1 Gbps 1 100 Mbps 1 10 Mbps 10 1.544 Mbps 64 768 Kbps 133 384 Kbps 266 128 Kbps 781
Table 1 - Default OSPF cost calculations.
There are two methods for changing the calculated costs - auto-cost and interface cost.
The auto-cost reference-bandwidth command allows you to change the reference bandwidth that OSPF uses to calculate its metrics:
router ospf 1
... auto-cost reference-bandwidth 1000000
REMEMBER You must ensure the reference bandwidth is consistent across all routers.
ip ospf cost can also be used to change the OSPF cost. However, this works on a per-interface basis and will override the calculated cost.
... ip ospf cost 1234
To verify the interface cost, the following command is used:
show ip ospf interface | inc Cost
Process ID 1, Router ID 184.108.40.206, Network Type BROADCAST, Cost: 1 Topology-MTID Cost Disabled Shutdown Topology Name Process ID 1, Router ID 220.127.116.11, Network Type BROADCAST, Cost: 1 Topology-MTID Cost Disabled Shutdown Topology Name