[colmmacc]

The default answer includes multiple A records, so if clients can't reach one of the IPs, they try another. There's no need for anything to propagate for that to kick in, it's just ordinary client retry behavior.

We do also withdraw an IP from DNS if it fails; when we measure it, we see that over 99% of clients and resolvers do honor TTLs and the change is effected very quickly. We've been using this same process for www.amazon.com for a long time.

Contrast to an alternative like BGP anycast, where it can take minutes for an update to propagate as BGP peers share it with each other in sequence.