The DNS age timer is a decrementing timer to zero and applies to a group/group range. The value of this timer is calculated by the following formula:
DNS age = (value of TTL in the DNS response) + (wait TTL)
wait TTL = 260 seconds
A DNS request is sent when receiving an MLDv1 report, but the TTL has expired. The source is not removed until the wait TTL time expires.
On receiving the DNS response, the existing DNS resolved addresses which are not in the DNS response are removed. Any new addresses in the DNS response are added and the DNS age timer is refreshed. The L2 SSM data caches are modified for the addresses removed, or added. PIM is notified to take action on L3 SSM caches.
If the DNS response is not received and the DNS age timer expires, the mapping entry is removed (if there are no static addresses). The SSM data traffic forwarding is stopped immediately when the group is removed.