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Looks like it was known broken at one point:
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The DNS Query Response TTL of the secondary DNS server is not being respected by the 9971/9951/8831 phones if the primary server goes down after the first request.<br class="">
Resolution Summary<span class="Apple-tab-span" style="white-space:pre"> </span>
<div class="">CSCut29536 for 8831 IP Phones </div>
<div class="">CSCut29519 for 99xx IP Phones </div>
<div class="">CSCut75647 for 8811/51 IP phones </div>
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<div class="">So versions and deeper dive required.</div>
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<div class="">-Wes</div>
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<div class="">On Apr 5, 2018, at 3:14 PM, Russell Goings <<a href="mailto:russgoings@rgoings.com" class="">russgoings@rgoings.com</a>> wrote:</div>
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<div dir="ltr" class="">Hi all
<div class=""> I have a client who was doing some network mucking around and part of that maintenance caused the two DNS servers used by a group of IP phones to go unavailable. Some of these phones went unregistered with a reason code 13 - TCP KeepAlive
Timeout. </div>
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<div class="">What I (OK not me. Just the client. Well maybe I am interested as well) would like to know is how do the Cisco SIP phones, particularly the 8851 model, deal with the TTL from the DNS response. </div>
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<div class="">Does the phone honor the DNS TTL and cache the response for 24 hours?</div>
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<div class="">Does the phone have a different cache time for the DNS response?</div>
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<div class="">Does it make a new DNS query every time it sends a keep-alive?</div>
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<div class="">I have never looked into this before and on this Thursday afternoon my Google-fu is weak.</div>
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<div class="">And the incident was over a week ago so not much from the phone logs to help.</div>
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<div class="">Thanks</div>
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<div class="">Russell Goings (AM)</div>
<div class="">IPT Engineer – Dimension Data</div>
<div class="">Direct Line 571-203-4021</div>
<div class=""><a href="mailto:russell.goings@dimensiondata.com" class="">russell.goings@dimensiondata.com</a></div>
<div class="">Monday – Friday 8:00 – 5:00 EST</div>
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