OSPFInterface Config

ip ospf cost [value]

Manually sets the OSPF cost (metric) on an interface, overriding the default cost derived from bandwidth, to influence path selection in OSPF routing.

Syntax·Interface Config

ip ospf cost [value]

When to Use This Command

Forcing traffic to prefer a higher-bandwidth link by setting a lower cost on that interface.
Making a backup link less preferred by assigning a higher cost to it.
Equalizing costs across unequal-bandwidth links for load balancing.
Adjusting OSPF metric to steer traffic away from a congested link.

Command Examples

Setting OSPF cost on a GigabitEthernet interface

Router(config-if)# ip ospf cost 10

This command sets the OSPF cost to 10 on the current interface. No output is generated; the change is applied immediately.

Verifying the configured OSPF cost

Router# show ip ospf interface gigabitEthernet 0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
  Backup Designated router (ID) 2.2.2.2, Interface address 192.168.1.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:03
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 25
  Last flood scan time is 0 msec, maximum is 4 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 2.2.2.2  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

The output shows OSPF interface details. The 'Cost: 10' field confirms the configured cost. Other fields: 'Internet Address' is the interface IP, 'Area' is the OSPF area, 'Process ID' and 'Router ID' identify the OSPF process, 'Network Type' is broadcast, 'State' is DR (Designated Router), and 'Neighbor Count' shows adjacent neighbors.

Understanding the Output

The 'show ip ospf interface' command displays OSPF parameters for an interface. The 'Cost' field shows the OSPF metric; lower values indicate more preferred paths. Default cost is 10^8 / bandwidth (bps). A manually set cost overrides this. Other key fields: 'State' indicates the router's role (DR, BDR, DROTHER); 'Neighbor Count' shows discovered neighbors; 'Adjacent neighbor count' shows fully adjacent neighbors. High cost values (e.g., >100) may indicate slower links or misconfiguration. Watch for mismatched costs on links that should be equal.

CCNA Exam Tips

CCNA exam tip: OSPF cost is inversely proportional to bandwidth; default cost = 10^8 / bandwidth. Manually setting cost overrides this.

CCNA exam tip: The 'ip ospf cost' command is configured in interface configuration mode, not OSPF router mode.

CCNA exam tip: OSPF uses cost as the metric; the lowest cost path to a destination is preferred.

CCNA exam tip: To verify the cost, use 'show ip ospf interface' and look for the 'Cost' field.

Common Mistakes

Mistake: Setting cost on the wrong interface, causing unintended path selection.

Mistake: Forgetting that cost is a 16-bit value (1-65535); values outside this range are rejected.

Mistake: Assuming cost is the only factor; OSPF also considers path type (intra-area, inter-area, external).

Related Commands

show ip ospf interface

Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.

show ip ospf neighbor

Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.

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