During a Disaster Recovery (DR) drill, PTF downloads to our iSeries servers were slow. Painfully slow. We asked the DR vendor to change the switch settings, check the cabling, etc. This did not help.
We then checked our TCP settings and soon after noticed our download speeds were much better. We had inadvertently cleared our DNS cache when we had opened the TCP menu (CFGTCP).
The restores from our SAVSYS tapes were months old and there had been DNS changes on the network in the interim that we had forgotten about.
Question:
Is there a way to clear the DNS resolver cache?
Resolution:
Using the CFGTCP command, select option 12. Pressing the Enter key will
clear the DNS resolver cache (sometimes referred to as the NAME cache).
http://archive.midrange.com/midrange-l/200801/msg00306.html
keyword: Slow PTF downloads.
harleysavage 
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