OSPF (Open Shortest Path First) is a link-state routing protocol that uses a cost metric to calculate the best route between two nodes. In this article, we'll look at how OSPF calculates costs and discuss some of the factors that can influence them.
So, if you're curious about how OSPF's cost calculation works or want to learn how to tweak its settings for your network, read on!
What is "OSPF Cost"?
OSPF cost is a metric used by the OSPF routing algorithm to calculate the best path between two OSPF-enabled devices. The cost of a path is determined by adding up the costs of the individual links that make up the path. The cost of a link is typically expressed in terms of bandwidth.
In most cases, the cost of a link is inversely proportional to its bandwidth. As a result, OSPF-enabled devices will typically prefer paths with high-bandwidth links.
How is OSPF Cost Calculated?
The OSPF cost is calculated based on the link's bandwidth and is used by the OSPF algorithm to determine the best path between two routers. The higher the bandwidth, the lower the cost. The lower the cost, the more preferable the link.
In most cases, the OSPF cost is calculated automatically by the router. However, sometimes, it may be necessary to set the OSPF cost manually. To do this, use the ip ospf costcommand. The value entered with this command will be used as the link's OSPF cost. Keep in mind that this value must be an integer between 1 and 65535. If it is not, the router will revert to using the default OSPF cost calculation.
The OSPF Cost is calculated by using this formula: reference bandwidth /interface bandwidth.
Here Are the Default Costs
Bandwidth - Cost
Gigabit Ethernet Interface (1 Gbps) - 1
Fast Ethernet Interface (100 Mbps) - 1
Ethernet Interface (10 Mbps) - 10
DS1 (1.544 Mbps) - 64
DSL (768 Kbps) - 133
A Little Problem
As you can see, OSPF considers all interfaces with a bandwidth of 100 Mbps or more as equal. The best possible cost is always 1. So no matter the speed of the internet, it will all have a default cost of 1 when it is over 100 Mbps. This can create subpar routing in up-to-date networks that use current high-speed ethernet interfaces.
It is possible to force a router to use a faster route manually. To change the reference bandwidth in an OSPF network, use the following command:
router ospf 1
Where is the desired reference bandwidth in Mbps. For example, to configure the reference bandwidth to 1 Gbps, use the following command:
By doing this, you can ensure that your routers will pick the fastest route possible.
You must to check CCIE Enterprise training about all useful commands of OSPF.
Under What Circumstances Might You Need to Manipulate OSPF Metrics?
In a typical OSPF network, the cost is automatically calculated based on the bandwidth of the link. However, there are times when it may be necessary to manipulate the cost metric manually. For example, if two links have different bandwidths, but the router considers them to be equally reliable, it may make sense to increase the cost of the lower bandwidth link to reduce traffic on that link.
In another example, if one link is consistently congested while another is not, it may be necessary to decrease the cost of the congested link to encourage more traffic to use that link. These are just a few examples of when it might be necessary to manipulate OSPF cost metrics; in general, any time there is an imbalance in traffic or reliability between two links, manipulating the cost metric can help to restore balance. You can learn more about OSPF costs and other topics on Cisco's Design Guide