OSPF link cost design

Architecturedesignospf

What kind of processes and/or tools do you use to design OSPF cost on links in your environments?

Best Answer

Not sure this is a question that can get a really good answer but it's important to know the preference order of OSPF:

intra area routes
inter area routes
external type 1
external type 2
NSSA type 1
NSSA type 2

So in some cases adjusting the metric has no effect on traffic engineering. It's important to change the auto-cost reference-bandwidth on Cisco devices so that there is a difference in cost between higher speed interfaces. By default this might be set to 100 Mbit. This would mean that 100 Mbit, gig and 10 gig interfaces would have the same cost.

For faster convergence it's good to have equal cost routes (ECMP) so if you are peering iBGP to a loopback you two paths and if one goes down the impact should not be that big.

I used to work for a large ISP and they set all the costs manually on the transit interfaces. The basic design was to choose the path that had the fewest core hops. The network was designed with access, distribution and core levels with dual links between each level. If there are multiple paths with same number of core hops then choose the path with the lowest delay.

So based on this they designed the paths and set costs on the interfaces. This might require a lot of planning. The bad thing about link state protocols is that they only use the bandwidth of the interface to calculate the metric. So you could have a case where the metric is the same but the distance is significantly longer via one of the paths.

We are talking quite large networks before this would have an impact though.

So I would design it based on bandwidth, number of devices to go through, physical distance (delay) and of course money could be a factor if one path is more expensive than the other.

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