Comment 39 for bug 1953165

Reviewed: https://review.opendev.org/c/openstack/neutron/+/880929
Committed: https://opendev.org/openstack/neutron/commit/071255f098e0e73fd5220f83cbbc8ac1c421f3ab
Submitter: "Zuul (22348)"
Branch: stable/2023.1

commit 071255f098e0e73fd5220f83cbbc8ac1c421f3ab
Author: Bence Romsics <email address hidden>
Date: Mon Mar 6 13:04:01 2023 +0100

    Suppress IPv6 metadata DAD failure and delete address

    IPv4 DAD is non-existent in Linux or its failure is silent, so we
    never needed to catch and ignore it. On the other hand IPv6 DAD
    failure is explicit, hence comes this change.

    This of course leaves the metadata service dead on hosts where
    duplicate address detection failed. But if we catch the
    DADFailed exception and delete the address, at least other
    functions of the dhcp-agent should not be affected.

    With this the IPv6 isolated metadata service is not redundant, which
    is the best we can do without a redesign.

    Also document the promised service level of isolated metadata.

    Added additional tests for the metadata driver as well.

    Conflicts:
        neutron/conf/agent/database/agentschedulers_db.py
            conflict with 831ac3152dc

    Change-Id: I6b544c5528cb22e5e8846fc47dfb8b05f70f975c
    Partial-Bug: #1953165
    (cherry picked from commit 2aee961ab6942ab59aeacdc93d918c8c19023041)