nanog mailing list archives

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


From: Owen DeLong <owen () delong com>
Date: Sat, 23 Oct 2010 07:45:23 -0700


On Oct 23, 2010, at 7:26 AM, Mark Smith wrote:

On Fri, 22 Oct 2010 15:42:41 -0700
Owen DeLong <owen () delong com> wrote:


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...)


No it doesn't. You can set the router lifetime to zero, which indicates
to the end-node that the RA isn't announcing a default router. In this
case, it may be announcing M/O bit, prefix or other parameters.

DHCPv6 can selectively give different information to different hosts
on the same wire segment.

RA cannot.


That was not the assertion you made.

You said that 

"The RA system assumes that all routers
capable of announcing RAs are default routers"

and I said, no, that is not the case if you set the RA lifetime to
zero. To cite explicitly, RFC4861 says,

Right... I oversimplified the point I was attempting to make and you
called me on it... Move on.

     Router Lifetime
                    16-bit unsigned integer.  The lifetime associated
                    with the default router in units of seconds.  The
                    field can contain values up to 65535 and receivers
                    should handle any value, while the sending rules in
                    Section 6 limit the lifetime to 9000 seconds.  A
                    Lifetime of 0 indicates that the router is not a
                    default router and SHOULD NOT appear on the default



Narten, et al.              Standards Track                    [Page 20]
^L
RFC 4861               Neighbor Discovery in IPv6         September 2007


                    router list.  The Router Lifetime applies only to
                    the router's usefulness as a default router; it
                    does not apply to information contained in other
                    message fields or options.  Options that need time
                    limits for their information include their own
                    lifetime fields.


I was not making any statements about whether DHCPv6 could be
selective about providing certain options to selected end-nodes.

You might think I'm being overlay pedantic, however changing the
question to then disagree with answer that doesn't agree with yours is
being disingenuous. 

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.


I wouldn't necessarily disagree, although in my experience they're
really quite rare, to the point where segmenting them into a separate
subnet, via e.g. a different VLAN, becomes a somewhat better and easier
option.

While I would agree with you operationally, sometimes they involve
software that discovers other devices by broadcast and does not
permit other mechanisms.

I've seen environments where they're able to deal with this in IPv4
because of this flexibility in DHCPv4 and would be limited to static
addressing in IPv6 because it lacks this ability.

Owen



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