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

Fri Oct 22 08:10:08 UTC 2010

On Oct 22, 2010, at 12:55 AM, Mark Smith wrote:

> On Fri, 22 Oct 2010 15:52:08 +1100
> Karl Auer <kauer at biplane.com.au> wrote:
> 
>> On Thu, 2010-10-21 at 21:05 -0500, Jack Bates wrote:
>>> On 10/21/2010 8:39 PM, Ray Soucy wrote:
>>>> 
>>>> How so? We still have RA (with a high priority) that's the only way
>>>> DHCPv6 works.  I guess there is a lot of misunderstanding about how
>>>> DHCPv6 works, even among the experts...
>>> 
>>> Actually, the last I checked, there are implementation of DHCPv6 without RA.
>> 
>> I'll go out on a limb here and say that RA is not needed for DHCPv6.
>> 
> 
> RAs are still needed to convey the M/O bit values, so that end-nodes
> know they need to use DHCPv6 if necessary. As there are two address
> configuration methods, there is always going to be a need to express a
> policy to end-nodes as to which one they need to use.
> 
You can actually force a client to assume the M bit if you cause it to launch
a DHCPv6 client through other means. You don't have to have RA for that.
Policy can be expressed by RA, or, it can be expressed by other means.

>> A DHCPv6 client multicasts all its messages to the well-known
>> all-relays-and-servers address. A client needs only its link-local
>> address to do this. The relay (or server if it happens to be on the same
>> link) can thus talk to the client in the complete absence of RA.
>> 
> 
> There isn't a method to specify a default gateway in DHCPv6. Some
> people want it, however it seems a bit pointless to me if you're going
> to have RAs announcing M/O bits anyway - you may as well use those RAs
> to announce a default router too.
> 
Actually, it's not pointless at all. The RA system assumes that all routers
capable of announcing RAs are default routers and that virtually all routers
are created equal (yes, you have high/medium/low, but, really, since you
have to use high for everything in any reasonable deployment...)

There are real environments where it's desirable to have a way to tell
different clients on a network to use different default gateways or
default gateway sets.

Owen