subnet prefix length > 64 breaks IPv6?

Jeff Wheeler jsw at
Wed Dec 28 22:39:55 UTC 2011

On Wed, Dec 28, 2011 at 5:07 PM, Ray Soucy <rps at> wrote:
> The suggestion of disabling ND outright is a bit extreme.  We don't
> need to disable ARP outright to have functional networks with a
> reasonable level of stability and security.  The important thing is

I don't think it's at all extreme.  If you are dealing with an access
network where DHCPv6 is the only legitimate way to get an address on a
given LAN segment, there is probably no reason for the router to use
ND to learn about neighbor L3<>L2 associations.  With DHCPv6 snooping
the router can simply not use ND on that segment, which eliminates
this problem.  However, this feature is not yet available.

It would also be difficult to convince hosting customers to use a
DHCPv6 client to populate their gateway's neighbor table.  However, if
this feature comes along before other fixes, it will be a good option
for safely deploying /64s without ND vulnerabilities.

> that we work with vendors to get a set of tools (not just one) to
> address these concerns.  As you pointed out Cisco has already been
> doing quite a bit of work in this area, and once we start seeing the
> implementations become more common, other vendors will more than
> likely follow (at least if they want our business).
> Maybe I'm just a glass-half-full kind of guy. ;-)

I think your view of the Cisco work is a little optimistic. :)  What
they have done so far is simply acknowledge that, yes, ND exhaustion
is a problem, and give the customer the option to mitigate damage to
individual interfaces / VLANs, on the very few platforms that support
the feature.

Cisco has also given the SUP-2T independent policers for ARP and ND,
so if you have a SUP-2T instead of a SUP720 / RSP720, your IPv4 won't
break when you get an IPv6 ND attack.  Unfortunately, there are plenty
of people out there who are running IPv6 /64s on SUP720s, most who do
not know that an attacker can break all their IPv4 services with an
IPv6 ND attack.

> The most important thing is that network operators are aware of these
> issues, have a basic understanding of the implications, and are
> provided with the knowledge and tools to address them.

We certainly agree here.  I am glad the mailing list has finally moved
from listening to Owen DeLong babble about this being a non-problem,
to discussing what work-arounds are possible, disadvantages of them,
and what vendors can do better in the future.

My personal belief is that DHCPv6 snooping, with ND disabled, will be
the first widely-available method of deploying /64s "safely" to
customer LAN segments.  I'm not saying this is good but it is a
legitimate solution.

Jeff S Wheeler <jsw at>
Sr Network Operator  /  Innovative Network Concepts

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