TTL question

[joe]

Hello,

 Which value determines the time a zones data will sit in a remote
servers cache. Is it the minimum SOA value or the TTL value. From
what I have read, the minimum value doesn't seem to get used for this.
If that is the case if I need to make changes to a zone and I adjust
the TTL period to a lower value temporarily (say 2 hours) is it safe
to say that my zone changes on remote DNS's would be updated
accordingly. Obviously I would need to adjust the TTL so that based on
its current value the "desired" value would take effect.

Also for the following example I performed a "dig + trace
www.cisco.com". How are the TTL's values (which I believe are the
second column values from the left) established ? I can understand
that the cisco.com zone would have a value of 86400 as setup by their
own DNS, but wouldn't that value propagate to the upper nameservers?
Why is the value different and how would subsequent changes be updated
to upper name servers ?

thanks kindly,
joe

dig +trace cisco.com

..                       55628   IN      NS      j.root-servers.net.
..                       55628   IN      NS      k.root-servers.net.
..                       55628   IN      NS      l.root-servers.net.
..                       55628   IN      NS      m.root-servers.net.
..                       55628   IN      NS      a.root-servers.net.
..                       55628   IN      NS      b.root-servers.net.
;; Received 324 bytes from 127.0.0.1#53(127.0.0.1) in 3 ms

com.                    172800  IN      NS      A.GTLD-SERVERS.NET.
com.                    172800  IN      NS      B.GTLD-SERVERS.NET.
com.                    172800  IN      NS      C.GTLD-SERVERS.NET.
com.                    172800  IN      NS      D.GTLD-SERVERS.NET.
com.                    172800  IN      NS      E.GTLD-SERVERS.NET.
com.                    172800  IN      NS      F.GTLD-SERVERS.NET.
com.                    172800  IN      NS      G.GTLD-SERVERS.NET.
com.                    172800  IN      NS      H.GTLD-SERVERS.NET.
com.                    172800  IN      NS      I.GTLD-SERVERS.NET.
com.                    172800  IN      NS      J.GTLD-SERVERS.NET.
com.                    172800  IN      NS      K.GTLD-SERVERS.NET.
com.                    172800  IN      NS      L.GTLD-SERVERS.NET.
com.                    172800  IN      NS      M.GTLD-SERVERS.NET.
;; Received 459 bytes from 192.33.4.12#53(c.root-servers.net) in 42 ms

cisco.com.              172800  IN      NS      ns1.cisco.com.
cisco.com.              172800  IN      NS      ns2.cisco.com.
;; Received 95 bytes from 192.5.6.30#53(A.GTLD-SERVERS.NET) in 45 ms

cisco.com.              86400   IN      A       198.133.219.25
cisco.com.              86400   IN      NS      ns1.cisco.com.
cisco.com.              86400   IN      NS      ns2.cisco.com.