Re: A common problem with SLAAC in "renumbering" scenarios

[Mark Smith <[e-mail]>]

On Sat, 16 Feb 2019 at 08:20, Sander Steffann <[e-mail]> wrote:
>
> Hi Lee,
>
> > Seems to me several operators have offered their opinions here. If you want more opinions, maybe ask a group of operators?
>
> Not a very helpful comment... I think the underlying problem here is that there is no technical solution available that solves stable IPv6 address delegation at residential scale in an acceptable way.
>
> - For stable address delegation the routing inside the ISP becomes a mess, especially when it can't be guaranteed that a customer consistently ends up at the same BNG.
> - To keep ISP networks scalable, prefixes are aggregated to (roughly) BNG level, making it impossible to keep prefix delegation stable at the customer's end
>

It's not a hard requirement to aggregate prefixes at the individual
BNG level, and most ISPs should be running BGP to carry customer
routes. Obviously BGP to scale to 100s of 1000s of routes because the
Internet runs with it.

The minimum PD stability needed for fixed access customers (e.g. ADSL,
FTTH etc.) is within the pool of BNGs they can attach to upon
reconnection. So if you need or want to aggregate those routes, you
place the BGP route aggregation boundary at the boundary of your BNG
pool, rather than at the individual BNG. You then say to your
customers that if they physically move their service by e.g. moving
house, they may not receive the same stable PD prefix again, because
you're not guaranteeing they will reconnect to the same BNG pool.

It would however be nice to try to provide the customer with a stable
prefix on a greater scale than just within the same BNG pool, so you
could pick greater geographic boundaries for your aggregation points,
rather than at the boundary of an individual BNG pool. For example,
here in Australia, I'd try to aim for that boundary to be at each of
the 7 capital city boundaries, even though I may have multiple PoPs
within a city, with each PoP containing a number of BNGs in a pool
(and possibly multiple BNG pools within a PoP). Smaller aggregation
domains e.g. dividing cities into 4 regions might be necessary
depending on numbers of subscribers.

You can still provide a guaranteed static and persistent PD prefix to
customers that want to pay for it, which means that prefix isn't ever
aggregated and can follow them regardless of where they geographically
because you never aggregate that prefix. They're paying for and
getting more value because they're getting a guarantee.



> With IPv4 this didn't cause massive problems because NAT disconnected the LAN addresses from the WAN side at the customer's end. With IPv6 this is no longer the case and it does cause problems.
>
> Stability at one end causes instability at the other end, and vice versa.
> And as can be seen from this discussion it's still an unsolved problem...
>
> Cheers,
> Sander
>
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