Re: dual stack & IPv6 on by default

[Tim Chown <tjc@ecs.soton.ac.uk>]

On Sun, Mar 16, 2003 at 07:38:09PM +0100, Erik Nordmark wrote:
> 
> A possible way to approach this problem would be to make
> the choice between A and AAAA be a function of whether there
> is one or more IPv6 route off-link (or at least one IPv6 router sending RAs).

Note this cuts both ways.  I've been in dual-protocol networks where IPv4
has been down and IPv6 up, so we ought to consider the general case of
availability of connectivity off-link for either protocol.

(But I do appreciate the issues here is - I think - what to do when deploying 
an IPv6 enabled host which receives AAAA responses in an IPv4-only network)

A not-uncommon enterprise deployment method for dual-protocol now is to have
a parallel IPv6 infrastructure with a separate off-site IPv6 link, such
that IPv6 is routed within the site using (for example) BSD routers with RA's
for subnets injected into the v4 VLANs (if your BSD box can VLAN-tag on a
single interface, very few cables may be needed :).  Thus the router giving 
connectivity off each subnet is different (in one case, the "smart" L2/L3 
VLAN switching equipment, in the other, PC-based routers) , and either could 
fail.  Such a deployment method is a short-term one until Cisco, Alcatel, etc
offer v6 L2/L3 functionality as exists for v4 now.

Most dual-protocol networks will probably in the medium-term fate-share IPv4 
and IPv6, so the distinction will then be somewhat moot.

Tim