Question

A new Cisco IP phone running SIP firmware is deployed at a branch site. The network supports both IPv4 and IPv6, but CUCM is configured for IPv4-only communication on its cluster-wide addressing settings. The phone obtains both an IPv4 and IPv6 address via DHCP and DHCPv6. The phone's Common Device Configuration is set to an IP Addressing Mode of 'IPv6 Only'. The phone fails to register with CUCM, and packet captures show the phone attempting to reach the TFTP and CUCM servers over IPv6 addresses that never respond. What is the MOST likely cause of the registration failure?

Accepted Answer

The phone's IP Addressing Mode in the Common Device Configuration conflicts with the cluster's IPv4-only communication setting, so the phone signals over IPv6 while CUCM services listen on IPv4.

Answer

The phone requires DNS SRV records for IPv6 that have not been created, preventing TFTP discovery.

Answer

The phone's SIP security profile is set to non-secure, which disables IPv6 transport.

Answer

The DHCPv6 scope is missing Option 150, so the phone cannot locate the TFTP server over IPv4.