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OSPF Cost, Interface Priority, and Passive Interfaces

CCNA: 0 to Hero - Comprehensive Network Engineering Bootcamp

Lesson 40: OSPF Cost and Optimization

OSPF Cost

OSPF uses Cost as its metric. The default calculation is:

$$\text{Cost} = \frac{10^8 \text{ (Reference Bandwidth in bps)}}{\text{Interface Bandwidth in bps}}$$

  • Default Reference Bandwidth is 100 Mbps, meaning Gigabit Ethernet links (1000 Mbps) also get a cost of 1, leading to suboptimal routing.

Adjusting Reference Bandwidth:

ios Router(config-router)# auto-cost reference-bandwidth 1000

(This sets the reference to 1000 Mbps, ensuring 1 Gbps links have a cost of 1 and 10 Gbps links have a cost of 0.1, rounded up to 1. Use 10000 for 10G links.)

Passive Interfaces

Interfaces connected to end-user segments (LANs) do not need to form OSPF adjacencies. Keeping OSPF disabled on these interfaces improves security and reduces processing overhead.

ios Router(config-router)# passive-interface GigabitEthernet 0/1

Note: The network connected to a passive interface is still advertised in OSPF.