Re: IPv6 fc00::/7 — Unique local addresses

Thu Oct 21 19:59:13 UTC 2010

I think you're misunderstanding how DHCPv6 works.  Don't think of it
like DHCP that you're used to.

DHCPv6 requires an IPv6 router advertisement to work.  There are three
flags of interest in a router advertisement.

One of them is the "A" (autonomous) flag which is enabled by default
in almost every implementation I've seen.  This is what signals a host
that it is permitted to use stateless configuration with the prefix.

There are also "M" (managed) and "O" other flags.  The "M" flag being
set signals the host that it should start a DHCPv6 client and make a
request for an address, the "O" flag signals that the host should ask
for "other" or additional configuration information through DHCPv6
(e.g. DNS servers).

None of the flags are exclusive, so you can enable DHCPv6 by setting
the M flag, but unless you disable the A flag, hosts will still use
stateless configuration (in addition to DHCPv6 and receive two
addresses)

If you want a DHCPv6-only environment, you simply disable the A flag
on the router advertisement.  This will stop hosts from using
stateless with the advertised prefix.

The default gateway for the network is learned through the router
advertisement, not through DHCPv6, which is why it doesn't exist in
DHCPv6.

Example IOS configuration:

interface Vlan123
 description Test IPv6 Network
 ipv6 address FD00:1234:5678:9ABC::1/64
 no ipv6 unreachables
 ipv6 nd prefix default 2592000 604800 no-autoconfig
 ipv6 nd managed-config-flag
 ipv6 nd other-config-flag
 ipv6 nd router-preference High
 no ipv6 redirects
 ipv6 verify unicast source reachable-via rx
 ipv6 eigrp 123
 ipv6 dhcp relay destination FD00:1234:5678:9ABC::2
 ipv6 dhcp relay destination FD00:1234:5678:9ABC::3

The "ipv6 nd prefix ... no-autoconfig" statement is what you're
looking for.  You need to type out timers to be able to get to it.
The values shown are just the Cisco defaults.

On Thu, Oct 21, 2010 at 3:43 PM, Luca Tosolini <bit.gossip at chello.nl> wrote:
> On Thu, 2010-10-21 at 14:19 -0400, Ray Soucy wrote:
>
>> We've decided to disable SLAAC (State-Less Address Auto-Configuration)
>> on almost all our IPv6 networks and use DHCPv6 exclusively.  This
>> allows us to only respond with DHCPv6 to the hosts we want to get an
>> IPv6 address instead of enabling it network-wide and crossing your
>> fingers.  The disadvantage here is that DHCPv6 client support is still
>> limited (OS X has none for example).   The argument is that IPv6 isn't
>> mission critical yet, so we're waiting to see if vendors will come
>> around and include DHCPv6 client support in the future.
>>
>
> Ray,
> how do you convey the default-router information with DHCPv6 only. AFAIK
> there is no such field in DHCPv6...
>
> Luca.
>
>

-- 
Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System
http://www.networkmaine.net/