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Thanks for your help, Vladimir. You are right, now that I look more closely, in dhcpv6-stateful mode the default gateway is acquired through router advertisement (instead of dhcp6 options). And the default gateway I get is the link-local address instead of the configured gateway_ip of 1:2:3:4::1.

This was confusing, to say the least - and not well explained anywhere - but it is working. I've tested all three modes of configuration (ipv6_address_mode and ipv6_ra_mode both simultaneously set to dhcpv6-stateful, dhcpv6-stateless or slaac). In all cases I am acquiring a default gateway - and it is the link local address of the default gateway. I can tell it works because the default gateway is pingable through the interface, and the low 3 bytes of the default gateway ip6 address matches the low 3 bytes of the gateway MAC address. (The MAC address is viewable when reviewing the router interfaces through the Horizon UI, for example - the actual link local ip6 addresses of the default gateway are not displayed).

Best,
Andrei


------------------------------

Message: 2
Date: Fri, 25 Dec 2015 19:07:31 +0300
From: Vladimir Eremin <veremin at mirantis.com>
To: "OpenStack Development Mailing List (not for usage questions)"
	<openstack-dev at lists.openstack.org>
Subject: Re: [openstack-dev] Configuring ISC dhclient on guest to
	acquire	ipv6 default gateway
Message-ID: <5864AF60-644C-438E-9D82-80C4FCED1137 at mirantis.com>
Content-Type: text/plain; charset=utf-8

Hi Andrei,

Default gateways for IPv6 is always configured with Router Advertisements (RA), no matter what addressing mode is used. Please ensure that:

- you have a virtual router connected to your IPv6 subnet, this would provide RA (and actual router) in your network
- accept_ra is enabled in your guest OS: sysctl -a | grep accept_ra

-- 
With best regards,
Vladimir Eremin,
Fuel Deployment Engineer,
Mirantis, Inc.