OSPF auto-cost reference-bandwidth 100000

Answered Question

Is this a 100 gig reference cost?

Are there any drawbacks to going this high?

I have this problem too.
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Correct Answer by Joseph W. Doherty about 7 years 10 months ago

As Giuseppe notes, the OSPF cost metric is a 16 bit value, and what can happen is slower links can overflow the metric cost. Remember, it's not just one link's metric, because the metric is cumulative from router to destination (depending on route types and area configuration).

From my experience, when the metic overflows it stays at the maximum value. What happens, if you have multiple paths to a destination, OSPF might not be able to distinguish a lower cumulative bandwidth path from a better path, i.e. they will look equal. (BTW: this is sort of the converse if you leave the default Cisco OSPF cost based on 100 Mbps, can't distiguish from 100 Mbps, from anything with higher bandwidth, e.g. multiple 100 Mbps Etherchannel, gig, etc.)

Correct Answer by Giuseppe Larosa about 7 years 10 months ago

Hello Andrew,

yes in this way an ip ospf cost 1 is associated to a 100GE link.

the link OSPF cost is an unsigned 16 bit quantity.

in this way a link with maximum cost 65535 is a link with bandwidth 1,5 Mbps.

So you haven't in your network links with speeds less then T1 you should be fine.

If there are their cost is the maximum possible or the lowest 16 bits of the ratio ?

I don't know.

If it is the lowest 16 bits it can be a problem that you can fix by setting manually the cost to 65534.

you can check with

sh ip ospf interface

to perform the test whatever interface is you can set an artifical low bandwidth then configure ospf with the autoreference-bw 1000000 and see what

sh ip ospf interface

tells about the cost.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 01/25/2009 - 12:34

Hello Andrew,

yes in this way an ip ospf cost 1 is associated to a 100GE link.

the link OSPF cost is an unsigned 16 bit quantity.

in this way a link with maximum cost 65535 is a link with bandwidth 1,5 Mbps.

So you haven't in your network links with speeds less then T1 you should be fine.

If there are their cost is the maximum possible or the lowest 16 bits of the ratio ?

I don't know.

If it is the lowest 16 bits it can be a problem that you can fix by setting manually the cost to 65534.

you can check with

sh ip ospf interface

to perform the test whatever interface is you can set an artifical low bandwidth then configure ospf with the autoreference-bw 1000000 and see what

sh ip ospf interface

tells about the cost.

Hope to help

Giuseppe

Correct Answer
Joseph W. Doherty Sun, 01/25/2009 - 17:06

As Giuseppe notes, the OSPF cost metric is a 16 bit value, and what can happen is slower links can overflow the metric cost. Remember, it's not just one link's metric, because the metric is cumulative from router to destination (depending on route types and area configuration).

From my experience, when the metic overflows it stays at the maximum value. What happens, if you have multiple paths to a destination, OSPF might not be able to distinguish a lower cumulative bandwidth path from a better path, i.e. they will look equal. (BTW: this is sort of the converse if you leave the default Cisco OSPF cost based on 100 Mbps, can't distiguish from 100 Mbps, from anything with higher bandwidth, e.g. multiple 100 Mbps Etherchannel, gig, etc.)

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