Internet Edge Router replacement - IPv6 route tablesizeconsiderations

Leo Bicknell bicknell at
Fri Mar 11 12:58:09 CST 2011

In a message written on Fri, Mar 11, 2011 at 01:07:15PM -0500, Valdis.Kletnieks at wrote:
> On Fri, 11 Mar 2011 09:38:12 EST, Joe Maimon said:
> > rfc3927 does not require 64 bits and works sufficiently well wherever it 
> > is employed. SLAAC should be redesigned to be configurable to work with 
> > however many bits are available to it and it should be a standard 
> > feature to turn that knob all the way from on - off with 128 bit stops 
> > in between.
> Feel free to explain how SLAAC should work on a /96 with 32 bits of host address
> (or any amount smaller than the 48 bits most MAC addresses provide).  Remember
> in your answer to deal with collisions.

Well, I at least think an option should be a /80, using the 48 bits
of MAC directly.  This generates exactly the same collision potential
as today we have with a /64 and an EUI-64 constructed from an EUI-48
ethernet address.  The router is already sending RA's for SLAAC to
work, sending along one of a well-known set of masks would be a
relatively minor modification.

That said, ND has built into it DAD - Duplicate Address Detection.
There is already an expectation that there will be collisions, and
the protocols to detect them are already in place.  I see little
to no reason you couldn't use a different length subnet (like the
/96 in your example), randomly select an address and do DAD to see
if it is in use.  Indeed, this is pretty much how AppleTalk back
in the day worked (with a 16 bit number space).

The probability of collision is pretty low, and the penalty/recovery
(picking a new address and trying again) is rather quick and cheap.

If a service provider is going to end up giving me a /64 at home (I
know, a whole different argument) I'd vastly prefer to use /80 or /96
subnets with either of these methods, and still be able to subnet the
space.  I suspect if /64's are given out one or both will come to be

       Leo Bicknell - bicknell at - CCIE 3440
        PGP keys at
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