Duplicate address detection

Duplicate address detection (DAD) is used to verify that an IPv6 home address is unique on the LAN before the address is assigned to a physical interface (for example, QDIO). z/OS® Communications Server responds to other nodes that are doing DAD for IP addresses assigned to the interface. DAD is not done for VIPAs or loopback addresses. DAD for local addresses is done for physical interfaces when one of the following situations occur:

To disable DAD checking, specify DUPADDRDET 0 on the INTERFACE statement.

DAD processing involves the following steps:
  1. The host joins a link-local all-nodes multicast group at interface start processing.
  2. The host joins a solicited-node group for the local address.
  3. A neighbor solicitation is sent to the solicited-node multicast address with the tentative address for which DAD is being performed.
  4. The host waits for a neighbor response (neighbor advertisement or neighbor solicitation) on the interface.
  5. If no neighbor response is received within the specified retransmit time, the address is considered unique on the LAN.
  6. If a neighbor response is received within the specified time, the address is not unique. The host leaves the solicited-node multicast group, issues a duplicated address detected console message, and marks the address unavailable because of a duplicate address.

Unless DAD is disabled, the address is not considered assigned to an interface until DAD is successfully completed for the local address. Packets can be received for the all-nodes or solicited-node multicast groups, but there is no response because the address is not yet assigned to the interface. If the local address is a manually configured address, the addresses are displayed in a Netstat Home/-h report as Unavailable (if the interface has not been started or if DAD failed).

In situations where DAD is not done for the IPv6 home address (by specifying DUPADDRDET 0 on the INTERFACE statement or if it is a VIPA), the z/OS Communications Server host still responds if another node is doing DAD for an IPv6 address assigned to the interface or for IPv6 VIPAs when the interface is assigned to handle VIPAs; responses are not sent for loopback addresses.