• Based on LSA cost associated with each link.
    • 0-65535
    • Best path is lowest end-to-end.
  • Default Cisco cost:
    • 100Mbps/Link bandwidth
    • 10Gbps links and higher need the auto reference bandwidth changed.
      • Ultimately cost is based off Reference Bandwidth / Interface bandwidth
  • Path Preference
    • Intra (O)
    • Inter Area (O IA)
    • E1
    • N1
    • E2
    • N2
      • Link cost does not matter in the order above.
      • Link cost only matters when multiple routes to same destination have same metric type.
  • Cost Modification:
    • interface bandwidth’
    • ‘Interface ip ospf cost’
    • process auto-cost’
    • ‘process neighbor cost’
  • Virtual Link Cost:
    • Inherit cost from SPF cost between Virtual Link Endpoints.
    • Must have cost below 65535 to initialize.
      • Higher than 65535 could occur if ref bandwidth is high and Virtual Link transits legacy links.
        • ex. Ref bandwidth is 40Gbps and VL transits a T1.

Interface OSPF Cost Value:

  • In the below image all routers are running area 0. R4 is advertising loopback IPs 1.1.1.1 and 2.2.2.2 into the area, and R2’s path to both advertised subnets is through R3 due to values over that link.

Looking at the OSPF costs of R2’s interfaces shows each with a cost of 1000 due to reference bandwidth set at 1000000.

If the cost command is applied to Gi0/3 as value 1 and Gi0/1 as 1001, we’ll see load balancing occur.

The reason for this is both paths will add up to the same metric to routes 1.1.1.1 and 2.2.2.2, value 2002.

Another way of changing this is through the Bandwidth command under the interface. This will most likely require the reference bandwidth changed, however.

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