Description of problem:
https://access.redhat.com/solutions/1467893

When sending an unsolicited Neighbour Advertisement when performing IP address failover on IPv6, there is no easy way to send an NA from addresses different than the link-local address.

The article recommends to run:
~~~
ndptool -t na -U -i vlan495 send
~~~

But this only sends out an advertisement for the link-local address:
~~~
(...)
8: eth1.905@eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2000:10::250/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::5054:ff:fe91:a4ec/64 scope link 
       valid_lft forever preferred_lft forever
(...)
 [root@undercloud-1 ~]# tcpdump -nne -ieth1.905 icmp6 &
[1] 21088
 [root@undercloud-1 ~]# tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1.905, link-type EN10MB (Ethernet), capture size 262144 bytes

 [root@undercloud-1 ~]#  ndptool -t na -U -i eth1.905 send
14:55:07.793683 52:54:00:91:a4:ec > 33:33:00:00:00:01, ethertype IPv6 (0x86dd), length 78: fe80::5054:ff:fe91:a4ec > ff02::1: ICMP6, neighbor advertisement, tgt is ::, length 24
~~~

We need an option `-s`, the same as in arping:
~~~
       -s source
              IP source address to use in ARP packets.  If this option is absent, source address is:

              · In DAD mode (with option -D) set to 0.0.0.0.

              · In Unsolicited ARP mode (with options -U or -A) set to destination.

              · Otherwise, it is calculated from routing tables.
~~~

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