What if DNS becomes unreachable

[billabrown78]

CUCM 12.5

I feel like this is something I should already know. But for some reason I do not. I'm curious whether SIP phones (78XX, 88XX) in a production environment will suddenly unregister with CUCM if DNS becomes unreachable. Does it store the DNS entry for the Subscriber IPs locally, or does it simply send the FQDN for translation by DNS every time the phone sends a keepalive?

Similarly, will the CUCM node syncs break?

[b.winter · Accepted solution]

Hi,

like in every system, the FQDN - IP relationship is cached for a certain amount of time (TTL) and after this, it will be deleted. And if the entry is deleted and the DNS-servers are not reachable, then the phones / servers will have connection problems.

This is not specific to CUCM / phone, this is how DNS entries and DNS in general work.

[billabrown78]

Very helpful. Thank you. Good info here on how to find out the DNS configured name cache TTL. 

https://www.cyberciti.biz/faq/how-to-see-time-to-live-ttl-for-a-dns-record/