OSPF Configuration

Reference Bandwidth: Interface cost = RefBW ÷ InterfaceBW. Raise RefBW for fast links (40G/100G+) to keep costs meaningful.

Definition

• RefBW is a global OSPF setting that defines how interface cost is computed.
• Lower cost = more preferred path.
• Interface Cost = Reference Bandwidth ÷ Interface Bandwidth

Default behavior

• Many platforms default RefBW to 100 Mbps.
• Good for 10/100 Mbps era, but collapses high-speed links to the same cost.

Link Speed	Formula	Cost
10 Mbps	100 / 10	10
100 Mbps	100 / 100	1
1 Gbps	100 / 1000	0.1 → rounded to 1

At ≥1 Gbps, all fast links look like cost 1, so OSPF can’t tell 1G from 10/40/100G.

Why raise RefBW

Use a larger RefBW so costs stay distinct at high speeds:

RefBW	Interface	Cost
100 Mbps	10 Gbps	0.01 → 1
10,000 Mbps (10 G)	10 Gbps	1
100,000 Mbps (100 G)	10 Gbps	10
100,000 Mbps (100 G)	40 Gbps	2.5
100,000 Mbps (100 G)	100 Gbps	1

Best practices

• Set the same RefBW on all OSPF routers to keep path math consistent.
• Pick a RefBW ≥ your fastest link (10,000; 100,000; or 1,000,000 Mbps).
• Cisco example:

  router ospf 1  auto-cost reference-bandwidth 100000   ! 100 Gbps

Analogy: RefBW is the ruler. If it’s only 100 Mbps long, all modern links look the same length. Use a longer ruler so OSPF can pick the fastest path.