Propagation · RFC 1034 · RFC 1996

DNS propagation

Changed a record and it does not show? We ask each DNS server of the zone directly and compare what they say, with the zone's serial on each, then show what our resolver still has cached and for how long.

What "propagation" means

It does not spread, it expires
The zone's servers have the new answer at once. Resolvers (providers', Google's, Cloudflare's…) keep the old one until its TTL runs out (RFC 1034 §3.6). If you know about a change in advance, lower the TTL beforehand, as the same RFC recommends.
The SOA serial
Each zone has a serial in its SOA that goes up with every change (RFC 1035 §3.3.13); the recommended form is YYYYMMDDnn (RFC 1912 §2.2). Secondary servers copy the zone when the serial rises: after a NOTIFY (RFC 1996) or when REFRESH runs out. A lower serial on a server means it has not copied it yet.
A non-authoritative answer
A server listed in NS must answer authoritatively for the zone. If it does not, that is a "lame" delegation (RFC 1912 §2.8): resolvers waste time on it or get wrong answers.
How we check
We find the zone and its NS servers through our resolver, then ask each IPv4 address directly, without recursion, only for the name and type asked plus the zone's SOA. Private, reserved or this server's addresses are not asked.

Sources: RFC 1034 §3.6 · RFC 1035 §3.3.13 · RFC 1912 · RFC 1996 · RFC 2308