How does OSPF pricing work?
OSPF relies on costs that are inversely proportional to the bandwidth of the link. Therefore, higher bandwidth links are preferred to lower ones. The Cost formula is reference bandwidth divided by interface bandwidth. The default reference bandwidth of 100 Mbps is used for OSPF cost calculation.
How does OSPF use the cost metric?
The OSPF metric for a route is the sum of the interface costs for all outgoing interfaces in the route. By default, a router's OSPF interface cost is actually derived from the interface bandwidth: The faster the bandwidth, the lower the cost.
What is the default metric calculation for 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 does OSPF increase cost?
Two methods are available to change the OSPF cost. One method is to run the ospf cost command to directly change the interface cost, and the other is to change the bandwidth reference value to indirectly change the interface cost.
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.
Does OSPF prefer higher cost?
OSPF prefers lowest cost path to the destination.
What is a cost metric?
Metrics are cost values used by routers to determine the best path to a destination network. Several factors help dynamic routing protocols decide which is the preferred or shortest path to a particular destination. These factors are known as metrics and algorithms.
What is the default cost of a serial interface with OSPF?
With the default of 100 Mbps, and default bandwidth, a serial interface's default OSPF cost is 100 / 1.544, or 64 (rounded down). Method 2 in this post changes the reference bandwidth, and leaves Fa0/0's interface bandwidth at 100 Mbps.
How does Eigrp calculate cost?
It is calculated by adding the advertised/reported distance advertised by the neighbor and the cost calculated by that current router to reach the neighbor. successor: It is the (lowest) best path to reach to any specific destination network.
How do I change bandwidth in OSPF?
To adjust the reference bandwidth, use the auto-cost reference-bandwidth Mb/s router configuration command. This command must be configured on every router in the OSPF domain. Notice that the value is expressed in Mb/s; therefore, to adjust the costs for: Gigabit Ethernet - auto-cost reference-bandwidth 1000.
OSPF Metric
The “path cost” is used as the metric for OSPF’s optimal route selection criteria. The costs on the path to the destination network are accumulated and the path with the lowest cost is adopted in the routing table as the optimal route. Think of it as measuring the distance to the destination network by path cost.
Example of path cost
To calculate the cumulative cost, OSPF routers run the SPF algorithm. For simplicity, assuming an OSPF route flows in, as in RIP, the cost of the interface receiving the route will be cumulative.
Changing OSPF Costs
You can control the determination of the optimal route by changing the OSPF cost. there are three ways to change the OSPF cost.
Verification of OSPF costs
To verify the cost of the OSPF, you can mainly use the following commands
OSPF Cost
OSPF Cost values are the values, that shows the feasibility of a link. So, how can we calculate an OSPF link’s cost? OSPF cost value is calculated with a formula that includes the bandwidh of the link and a reference bandwidh value.
OSPF Shortest Path First Algorithm
One of the important Routing Algorithms is Dijkstra Shortest Path First Algorithm in CCNP ENCOR Lesson. Shortest Path First is the algorithm that is used by OSPF Routing Protocol. Let’s explain the logic of shortest path first algorithm with an example. In the below example, there are three paths to go from Router A to Router F.
