Last Updated on August 7, 2021 by Admin 3
You administer the OSPF network shown in the diagram above.
The reference bandwidth has been changed to 1000 on every router in the network.
What is the cost of the route from RouterA to RouterC? (Select the best answer.)
In this scenario, the cost of the route from RouterA to RouterC is 3. In an Open Shortest Path First (OSPF) network, a cost is associated with every link on the network. The OSPF routing process on each router calculates the optimal route to other routers in the network based on the sum of the link costs to those routers. The route with the lowest cost is considered the best. If there is more than one route with the same cost, then the OSPF routing process will use load balancing to distribute traffic evenly among the routes. The cost of each link and the optimal route from RouterA to RouterC are shown in the following exhibit:
The cost of a link is based on the interface bandwidth and the reference bandwidth, as indicated by the following formula:
cost = reference bandwidth / interface bandwidth
The default reference bandwidth is 100 Mbps. If a bandwidth has not been configured on an interface, the OSPF process will use the default value for the interface type. For example, a 100Mbps Fast Ethernet interface has a default interface bandwidth of 100. The minimum supported cost for an OSPF interface is 1, and any values that calculate to less than 1 are rounded up to 1. Therefore, any link with an interface bandwidth greater than or equal to 100 Mbps will result in a cost of 1 by default. In this scenario, the reference bandwidth is 1000 Mbps. Thus a FastEthernet interface will have a cost of 10, and a 1Gbps GigabitEthernet interface will have a cost of 1.
An OSPF process uses cost values to generate its shortest path first (SPF) tree and then to determine the optimal routes to all known networks. Because the minimum cost value is 1, the reference bandwidth should be a value greater than or equal to the bandwidth of the fastest routed link in the administrative domain. If the reference bandwidth is less than the fastest routed link on the network, a situation can arise where the costs of two interfaces are the same even though their link speeds are different. For example, if the reference bandwidth in this scenario were reset to its default value, the cost of every link would have a value of 1. Because all links would then appear to have the same cost, the OSPF routing process would not be able to distinguish between the FastEthernet and GigabitEthernet links in the network. The OSPF process would then perform equalcost load balancing to distribute packets evenly among the available paths. This distribution would cause some packets in this example to take suboptimal routes to their destinations.
You can issue the autocost command from router configuration mode to change the reference bandwidth for an OSPF routing process. The syntax for the autocost command is autocost reference-bandwidth ref-bw, where ref-bw is an integer between 1 and 4294967. Alternatively, you can manually configure a cost at the interface level by issuing the ip ospf cost command.