From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: why are IPv6 addresses removed on link down
Date: Mon, 12 Jan 2015 23:10:21 -0800
Message-ID: <20150112231021.316648e3@urahara>
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To: David Ahern <dsahern@gmail.com>
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On Mon, 12 Jan 2015 22:06:44 -0700
David Ahern <dsahern@gmail.com> wrote:

> We noticed that IPv6 addresses are removed on a link down. e.g.,
>    ip link set dev eth1
> 
> 
> Looking at the code it appears to be this code path in addrconf.c:
> 
>          case NETDEV_DOWN:
>          case NETDEV_UNREGISTER:
>                  /*
>                   *      Remove all addresses from this interface.
>                   */
>                  addrconf_ifdown(dev, event != NETDEV_DOWN);
>                  break;
> 
> IPv4 addresses are NOT removed on a link down. Is there a particular 
> reason IPv6 addresses are?
> 
> Thanks,
> David

See RFC's which describes how IPv6 does Duplicate Address Detection.
Address is not valid when link is down, since DAD is not possible.