[cisco-voip] Looking for insight into how Cisco 8851 deals with DNS TTL

Fri Apr 6 10:01:51 EDT 2018

Looks like it was known broken at one point:

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.
Resolution Summary
CSCut29536 for 8831 IP Phones
CSCut29519 for 99xx IP Phones
CSCut75647 for 8811/51 IP phones

So versions and deeper dive required.

-Wes

On Apr 5, 2018, at 3:14 PM, Russell Goings <russgoings at rgoings.com<mailto:russgoings at rgoings.com>> wrote:

Hi all
   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.

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.

Does the phone honor the DNS TTL and cache the response for 24 hours?

Does the phone have a different cache time for the DNS response?

Does it make a new DNS query every time it sends a keep-alive?

I have never looked into this before and on this Thursday afternoon my Google-fu is weak.

And the incident was over a week ago so not much from the phone logs to help.

Thanks

Russell Goings (AM)
IPT Engineer – Dimension Data
Direct Line 571-203-4021
russell.goings at dimensiondata.com<mailto:russell.goings at dimensiondata.com>
Monday – Friday 8:00 – 5:00 EST

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