But it is, you can probe and route around brownouts in certain AS's in the path. I would say that a CDNs ability to get to the content to the eyeballs is the most important and trumps all else.
I think cost is most important. I know other CDNs use routes for managing costs too. Being able to modify traffic flows means you control the flow of money between peers and/or customer=>transit relationships.
Cost is unlikely to be most important. If it were the CDN could just buy the cheapest transit, accept a default route and call it a day.
In the article Fastly has 13 transit links at one site and the ability to ping each route to determine the best path. They would hardly go to all this trouble if cost control was the main objective.
Cost is important to any business, but CDNs are all about speed, the biggest metric is "time to first byte." This is always going to take precedence over cost for a CDN. Differences in transit costs for a large CDN with high commits is negligible.
There are more costs involved than just those of the CDN.
Let's say I'm a network operator. I have some number of POPs/facilities, peering relationships, transit customers maybe even some relationships where I pay for transit. If I decide to partner with or become a customer of a CDN who can control the path traffic takes to reach the CDN on my network, I/they/we can use that negate, or elminate, my transit costs AND drive revenue to me.
(very simple example) Lets say one of my transit customers also has a non-paid peering relationship with one of my upstreams. Customer may choose to take the longer path to reach the CDN on my network because it's free to them, rather than paying me. Now, not only are they not paying me for transit, but I may be incurring additional cost to pay my transit. If this routing data is in the CDN application, a logical step is to report on costs of the traffic. I would see this traffic taking a more costly path and be able to act on it. Maybe that means pre-pending my ASN, maybe that means modifying the DNS requests made from my customer's IP range(s) to use a block not routed across the other ASN. Maybe it means I simply don't want that traffic to my "nodes" anymore.
Maybe all anyone needs is "fast" today, I agree its not the only factor, but being able to manage cost was a discussion point in the past.
