IPv6 fc00::/7 — Unique local addresses

Mark Smith nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org
Sat Oct 23 09:26:29 CDT 2010

On Fri, 22 Oct 2010 15:42:41 -0700
Owen DeLong <owen at 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,

      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]
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


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