CCNA Training – OSPF Costs

As a CCNA / CCNP candidate you might be anticipated to know the way to set and interpret the OSPF price perform in your Cisco units

Throughout your profession as a Cisco community engineer you’ll have to take care of setting and manipulating the OSPF prices on an interface.

OSPF makes use of a metric referred to as “Value” to calculate the metric of path. The price is a cumulative worth which is an incremental metric.

The price is as a default based mostly on the bandwidth of the interface. The Increased the interface bandwidth the decrease the price that’s related to that interface, to see the price that’s assigned to any given interface which is collaborating in OSPF challenge the next command:

Router# present ip ospf interface

The output of this command will present the present price given to this interface. The prices of the interface is calculated by taking the bandwidth of the interface and dividing this quantity by a price often called the “auto-cost reference-bandwidth”. This auto-cost reference-bandwidth is an integer used to calculate a normal metric throughout OSPF and is about to 100,000,000. The price is calculated as follows:


If the interface bandwidth is 10Mbps, then the ensuing price could be 10:

100,000,000/10,000,000 = 10

If the interface bandwidth is 100Mbps, then the ensuing price could be 1:

100,000,000/100,000,000 = 1

The subsequent instance reveals some deficiencies with the auto-cost reference-bandwidth set at 100,000,000. If we had a 1Gbp interface it is price will likely be calculated by OSPF to be the very same quantity as that for the 100Mbps interface.

100,000,000/1000,000,000 = 1

The identical would apply for greater velocity interfaces equivalent to 10Gbps interfaces which might even be interpreted by OSPF as having a price of 1, since OSPF can not outline interface prices as an integer a part of a decimal quantity i.e. 0.5 or 0.125.

In order that OSPF might be able to calculate the price of an interface based mostly on the bandwidth of the interface with nice accuracy we might want to change the worth of the auto-cost reference-bandwidth worth from 100,000,000 to a larger worth.

To vary the ACFB use the next instructions:

router(config)#router ospf 1

R2_4(config-router)#auto-cost reference-bandwidth 10000

% OSPF: Reference bandwidth is modified.

Please guarantee reference bandwidth is constant throughout all routers.

The auto-cost reference-bandwidth now has a price of 10000,000,000 (The worth is about in Mbps), due to this fact now the OSPF course of will view the 100Mbps interface as having a price of 100, 1Gbps having a Value of 10 and 10Gbps a price of 1, one other level value mentioning is that the router will immediate you to make it possible for the auto-cost reference-bandwidth worth is about persistently throughout all of your routers.

Be aware: The price given to a path is added to the metric solely when the route is acquired inbound on an interface, not outbound, due to this fact do you have to want to alter the price of the trail you need to do that on the inbound interface.

Leave a Comment