OSPF Cost calculation

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Open Shortest Path First (OSPF) is a widely used interior gateway protocol in computer networking for routing IP packets within a single routing domain. One of the key concepts in OSPF is the calculation of the cost (metric) associated with each route, which is used to determine the shortest path to a destination. The cost is calculated based on the bandwidth of the interfaces involved in the route.
OSPF Cost Calculation
1. Reference Bandwidth
The reference bandwidth is a configurable value that represents the bandwidth of the highest-speed link in the network. It is used to normalize the OSPF cost calculation across different types of links. The default reference bandwidth in OSPF is 100 Mbps, but it can be changed to accommodate faster links, such as gigabit or multi-gigabit Ethernet.
2. Interface Bandwidth
The interface bandwidth is the actual speed of the link between routers. This is typically the bandwidth that the hardware can handle and is often configured on the router interface.
3. Cost Calculation Formula
The cost for an interface is calculated using the formula:
OSPF Cost
=
Reference Bandwidth
Interface Bandwidth
OSPF Cost=
Interface Bandwidth
Reference Bandwidth
​
The result is an integer value that represents the cost of using that interface for routing. A lower cost indicates a higher priority link.
4. Configuring OSPF Cost
You can manually configure the OSPF cost for an interface or let OSPF automatically calculate it based on the reference and interface bandwidths.
Example Calculation
Let's assume the reference bandwidth is set to 1000 Mbps (1 Gbps) and we have the following interface bandwidths:
Ethernet (10 Mbps)
Fast Ethernet (100 Mbps)
Gigabit Ethernet (1000 Mbps)
Using the default reference bandwidth (100 Mbps), the costs would be: