OSPF Bandwidth determination

Unanswered Question
Apr 22nd, 2007

According to Cisco:

"In Cisco IOS 10.3 and later, OSPF by default now calculates the cost (metric) for an interface according to the bandwidth of the interface."

How is the Bandwidth on the interface determined?

I have this problem too.
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CSCO10899265 Sun, 04/22/2007 - 16:57

Hi,

The cost of a link is calculated by dividing the reference cost to the bandwidth of an interface. The reference value by default is 10^8. For a fastethernet running at 100mbps, the cost is calculated as 10^8/1000000bps=10(cost of the link). alternatively the bndwidth can be set using the bandwidth command.

Richard Burts Sun, 04/22/2007 - 17:23

Richard

Each type of interface on a Cisco router has a default value for bandwidth of each of its interfaces. Many of them are intuitive and make sense (ISDN BRI defaults to 64K, Ethernet defaults to 10 Mb, FastEthernet defaults to 100 MB), some may be a bit less intuitive but still make sense (most serial interfaces default to 1.544 Mb (T1 speed)), and some seem a bit strange (GRE tunnels default to 9K). It is possible to over-ride the default bandwidth and configure a bandwidth value on the interface if you do not like the default.

It may also be worth pointing out that the bandwidth of the interface is a logical assignment and there is no direct dependency between the bandwidth of the interface and its operating speed. So changing the bandwidth of an interface will not change its real speed or throughput.

HTH

Rick

wilson_1234_2 Sun, 04/22/2007 - 18:34

So,

If I had two paths available to a remote site, one via the serial interface and the other via the fastethernet and wanted to change the desirable path to be the serial interfce, I would change the bandwidth setting on one or the other interface?

The change making the serial indicate a faster bandwidth than the fastethernet?

Richard Burts Sun, 04/22/2007 - 18:54

Richard

Changing the bandwidth of the interface would be one way to do it. There is another alternative which I believe might be even better. There is an option to configure the OSPF cost of an interface on the interface. In your example you could configure the cost of the serial to be lower or configure the cost of the FathEthernet to be higher and influence the path chosen.

I believe that this alternative is better because it achieves the result you want (select the path over the serial) without potential impact on any other function. While the bandwidth does not control the speed of the interface it is used for a few things. In addition to protocols like OSPF and EIGRP which use bandwidth to calculate a metric, bandwidth is used by IOS to calculate the load on the interface, it may be used by various types of net management software which may report link utilization. If you change the path chosen by configuring OPF cost then you achieve your objective without potential impact on these other things.

HTH

Rick

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