Two Stacks (IPv4 and IPv6) Overview
When your SIP network is configured for both IPv4 and IPv6 stacks, SIP devices can handle calls for each of the following scenarios:
All devices in the call support IPv4 only
All devices in the call support IPv6 only
All devices in the call support both IPv4 and IPv6 stacks. In this scenario, the system determines the IP address type by the configuration for the IP Addressing Mode Preference for Signaling setting for signaling events and the IP Addressing Mode Preference for Media enterprise parameter for media events.
One device supports IPv4 only and the other supports IPv6 only. In this scenario, Unified Communications Manager inserts an MTP into the call path to translate the signaling between the two addressing types.
For SIP devices and trunks, you can enable two-stack support by configuring Alternate Network Address Types (ANAT). When ANAT is applied to a SIP device or trunk, the SIP signaling that the device or trunk sends includes both an IPv4 and IPv6 address, if both are available. ANAT allows the endpoint to interoperate seamlessly in both IPv4-only and IPv6-only networks.