IPv6: OSPF Cost

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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. [2] 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.

int gi0/1

 ...
 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 1.1.1.1, Network Type BROADCAST, Cost: 1
 Topology-MTID    Cost    Disabled    Shutdown      Topology Name
 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
 Topology-MTID    Cost    Disabled    Shutdown      Topology Name