network cisco ccna gns3 certification arteq

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Friday, October 19, 2012

ospf cost...

cisco defines ospf cost as the inverse of bandwidth, whatever that means...
here is a great link from train signal, however, it doesn't explore cost as the inverse of bandwidth, it simply describes how to configure cost...

As already mentioned, the OSPF process assigns cost values to interfaces based on the inverse of the bandwidth parameter assigned to the interface with the bandwidth command.
For calculating the SPF to a given destination, the router takes into consideration the costs of the links along various paths. The path with the lower cost is selected as the shortest path.


Q. How does OSPF calculate its metric or cost?

A. OSPF uses a reference bandwidth of 100 Mbps for cost calculation. The formula to calculate the cost is reference bandwidth divided by interface bandwidth. For example, in the case of Ethernet, it is 100 Mbps / 10 Mbps = 10.
Note: If ip ospf cost cost is used on the interface, it overrides this formulated cost. For more information, refer to OSPF Cost.


The cost (also called metric) of an interface in OSPF is an indication of the overhead required to send packets across a certain interface. The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost. There is more overhead (higher cost) and time delays involved in crossing a 56k serial line than crossing a 10M ethernet line. The formula used to calculate the cost is:
  • cost= 10000 0000/bandwith in bps

and from scott morris, the simple answer:

So "auto-cost reference-bandwidth 100" is the default where 100M links = 1.

So for 1G link to equal 1, the auto-cost would be 1000.  For 10G, it would be 10000, for 100G, it would be 100000.

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