DNS

Cisco CUCM, Cisco IP phones, and other devices in a network can use DNS to resolve names to IP addresses. If the DNS server is down or inaccessible, or if the DNS entry is incorrect, failures can occur. For example, it is possible for Cisco IP phones to use DNS during registration, but registration will fail if they are unable to resolve the name of the CUCM.

Because DNS servers can be a point of failure in a network, it is a good idea to explicitly configure CUCM servers and other devices to use IP addresses whenever possible. To use IP addresses instead of DNS in a CUCM cluster, navigate to System, Server in Cisco CUCM Administration.

Although the use of DNS generally is not recommended for use in a Cisco CUCM environment, there are certain exceptions to this rule. For example, if a NAT device is between Cisco IP phones and the CUCM to which they are registered, DNS is required to ensure proper communication. Also, DNS can be used to provide disaster recovery for IP telephony in certain situations.