CISCO CCNA 200-301 Q123

Which metric does the Open Shortest Path First (OSPF) routing protocol use for optimal path calculation?

B. Cost
C. Delay
D. Hop count

Correct Answer: B

OSPF is a link-state routing protocol which uses cost as a metric for optimal path calculation. It is an open standard protocol based on Dijkstra’s Shortest Path First (SPF) algorithm. Metrics are used by routing protocols to determine the lowest cost path to a network number, which is considered the optimal or “fastest” path.

Cisco’s implementation of OSPF calculates the cost (metric) of a link as inversely proportional to the bandwidth of that interface. Therefore, a higher bandwidth indicates a lower cost, and a more favorable metric.

For this to work properly, the bandwidth of the link must be configured to allow OSPF to arrive at the cost of the link. This is done with the bandwidth command executed in interface configuration mode, and is entered in kbps. For example, if the link were 64 kbps, you would enter the following command:

Router(config-if)# bandwidth 64

The metric for any OSPF link defaults to 100,000,000/bandwidth. The bandwidth used in the formula is in bits per second. So, in this example the calculation would be 100,000,000 / 64000 = 1562.5. The cost assigned to the link would be 1562. The cost for a network route is the sum of all individual links in the path to that network.

If multiple paths are assigned equal costs, OSPF will load balance across the multiple paths. By default, it will limit this load balance to a maximum of four equal- cost paths. When this occurs, all four equal-cost paths will be placed in the routing table. There are two approaches to allow or prevent load balancing when multiple equal cost paths are available:

– Use the bandwidth command to make one or more of the paths either less or more
– Use the ip ospf cost command to change the cost value assigned to one or more of the paths

Maximum Transmission Unit (MTU), bandwidth, delay, load, and reliability form a composite metric used by Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP). IGRP is a distance vector routing protocol developed by Cisco Systems. Enhanced IGRP (EIGRP) is a Cisco-proprietary hybrid protocol having features of both distance-vector and link-state protocols.

Hop count is a metric used by Routing Information Protocol (RIP). The fewer hops between the routers, the better the path.