ttl delegations question

[Matus UHLAR - fantomas]

Hello,

there are some domains that have TTL for their own records lower than TTL
for glue records in their superior domains. e.g.:

uhlar at fantomas% dig ns documentum.com. @a.gtld-servers.net
[...]
documentum.com.         172800  IN      NS      ns10.documentum.com.
documentum.com.         172800  IN      NS      ns11.documentum.com.
[...]
ns10.documentum.com.    172800  IN      A       198.182.5.170
ns11.documentum.com.    172800  IN      A       198.182.5.174

uhlar at fantomas% dig ns documentum.com. @198.182.5.170
[...]
documentum.com.         3600    IN      NS      ns10.documentum.com.
documentum.com.         3600    IN      NS      ns11.documentum.com.
[...]
ns10.documentum.com.    3600    IN      A       198.182.5.170
ns11.documentum.com.    3600    IN      A       198.182.5.174

I'd like to ask, if BIND searches for e.g. MX record of documentum.com.
domain, which TTL does it use? 

in first lookup, bind receives NS and A records with TTL 172800,
in second lookup it receives the same NS and A records with TTL 3600.

I'd like to know, which ttl is used in both cases? is it always the
longer/shorter? Does TTL from zone override the glue TTL? 

Does this behaviour differ between different BIND versions?

-- 
Matus UHLAR - fantomas, uhlar at fantomas.sk ; http://www.fantomas.sk/
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