Many '...query: . IN NS' in query log file

[Nicolas Ecarnot]

David Botham <DBotham at OptimusSolutions.com> wrote in
news:ck1i5t$lso$1 at sf1.isc.org: 

>> client 81.206.200.71#1026: query: . IN NS
> 
> I am fairly certain this log entry indicates that the computer at IP 
> address 81.206.200.71 is asking your name server for the NS RR's for
> the root zone.

It seems clear, but is this normal that those servers asks that every ten 
second ?
Aren't they supposed to keep a cache of this kind of information ?

-- 
Nicolas Ecarnot