Some CDNs (like Cloudflare) use solely BGP anycast steering for routing to the "nearest" server. Other CDNs (like Akamai, Fastly, Netflix, and YouTube) use a hybrid BGP-DNS steering because some ISPs have extremely questionable routing practices.
Unfortunately, if the CDN only rely on BGP steering (or conversely if you are a user who is stuck on an ISP monopoly), there are cases where this is not necessarily the nearest network-wise (or performant network-wise) if there are peering disputes. If the said ISP is a virtual monopoly or (worse) state-sanctioned to collect network "toll fees" (like in South Korea), non-preferred and international routes are (intentionally) congested.*
If you use a third-party DNS, you basically lose this DNS optimization, and ECS does not fully solve this (because sometimes the DNS override are placed only on the ISP's recursive DNS servers). You're basically in a lose-lose position: either use third-party servers and the IP addresses served to you on popular CDNs are in the congested path, or use the often-unreliable and heavily-logged ISP-provided DNS.
* Usually. There are exceptions, but this comment is just a simplification of the complexities of real-life networking (where RFCs and mutual cooperation die out without fanfare).
alt Hacker News
Some CDNs (like Cloudflare) use solely BGP anycast steering for routing to the "nearest" server. Other CDNs (like Akamai, Fastly, Netflix, and YouTube) use a hybrid BGP-DNS steering because some ISPs have extremely questionable routing practices.
Unfortunately, if the CDN only rely on BGP steering (or conversely if you are a user who is stuck on an ISP monopoly), there are cases where this is not necessarily the nearest network-wise (or performant network-wise) if there are peering disputes. If the said ISP is a virtual monopoly or (worse) state-sanctioned to collect network "toll fees" (like in South Korea), non-preferred and international routes are (intentionally) congested.*
If you use a third-party DNS, you basically lose this DNS optimization, and ECS does not fully solve this (because sometimes the DNS override are placed only on the ISP's recursive DNS servers). You're basically in a lose-lose position: either use third-party servers and the IP addresses served to you on popular CDNs are in the congested path, or use the often-unreliable and heavily-logged ISP-provided DNS.
* Usually. There are exceptions, but this comment is just a simplification of the complexities of real-life networking (where RFCs and mutual cooperation die out without fanfare).
Edit for further reading: DNS is the new BGP by Geoff Huston of APNIC (https://ispcol.potaroo.net/2023-09/service-routing.html), How LinkedIn used PoPs and RUM to make dynamic content download 25% faster from the old LinkedIn engineering team (Archived at https://web.archive.org/web/20160310065302/https://engineeri...), Wikimedia's mapping of their CDNs (https://gerrit.wikimedia.org/r/plugins/gitiles/operations/dn...)