How is OSPF cost calculated?
OSPF uses the following formula to calculate the path cost when interface bandwidth is available: OSPF path cost = ((auto-cost × reference-bandwidth + interface bandwidth) -1) / interface bandwidth.
How does Cisco router calculate OSPF cost?
Formula for OSPF cost calculation is:
- Interface Cost= Reference bandwidth/interface bandwidth.
- Hence By default Interface Cost= 10^8/(interface bandwidth)
- The “auto-cost reference-bandwidth” command allows you to change the reference bandwidth that OSPF uses to calculate its metrics:
- Router#configure terminal.
What is the default cost of OSPF?
By default, OSPF assigns a default cost metric of 1 to any link faster than 100 Mbps, and a default cost metric of 0 to the loopback interface (lo0). No bandwidth is associated with the loopback interface. This means that all interfaces faster than 100 Mbps have the same default cost metric of 1.
How OSPF calculate the best route?
OSPF uses SPT tree to calculate the best route for routing table. A SPT tree cannot grow beyond the area. So if a router has interfaces in multiple areas, it needs to build separate tree for each area. SPF algorithm calculates all possible routes from source router to destination network.
What is cost metric in OSPF?
Open Shortest Path First (OSPF) uses “Cost” as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation. The formula to calculate the cost is Reference Bandwidth divided by interface bandwidth. For example, in the case of 10 Mbps Ethernet , OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10.
How do I change OSPF cost?
To explicitly specify the cost of an interface, issue the ip ospf cost command in interface configuration mode. To control how OSPF calculates the default metrics for the interface (based on the reference bandwidth), issue the auto-cost command in OSPF router configuration mode.
Does OSPF prefer higher or lower cost?
OSPF uses costs which are inversely proportional to the bandwidth of the link. As a result, higher bandwidth links are preferred over lower.
How does OSPF calculate routes?
OSPF is a dynamic link-state routing protocol that uses a link-state database (LSDB) to build and calculate the shortest path to all known destinations. It is through the use of Dijkstra’s SPF algorithm that the information contained within the LSDB is calculated into routes.
What is the default Cisco OSPF cost for Gigabit Ethernet?
The according to the default OSPF metric Cost value calculation, the default OSPF Cost for Fast Ethernet interface (100 Mbps) and a Gigabit Ethernet interface (1 Gbps) are same….What is OSPF Metric value Cost and OSPF default Cost Reference Bandwidth.
| Bandwidth | OSPF Cost |
|---|---|
| 10 Mbps | 10 |
| 1.544 Mbps | 64 |
| 768 Kbps | 133 |
| 384 Kbps | 266 |