To confirm that we can reduce the amount of time needed to move CDN request routing away from eqiad by lowering DNS TTLs, verify that ATS backends honor TTLs appropriately.
Apache Traffic Server can use DNS TTLs, a hardcoded setting, or a combination of the two depending on the value of proxy.config.hostdb.ttl_mode. See the documentation for further details, but long story short we use the default value (respect TTL from the DNS response).
This is the theory, and we have this task to verify that practice matches it.