v6 & DSL / Cable modems [was: Private use of non-RFC1918 IP space (IPv6-MW)]

Bill Stewart nonobvious at gmail.com
Sat Feb 7 19:17:08 CST 2009


On Fri, Feb 6, 2009 at 7:12 PM, Matthew Moyle-Croft
<mmc at internode.com.au> wrote:
> Jack Bates wrote:
> >  Dynamic or static; how does this alter the state of the routing table?...
> Dynamic assigned addresses mean that the BRAS the customer terminates on can
> hand out a range out of a pool assigned to it.  This means I can have a
> single route in my routing table for a whole BRAS (maybe 20k customers) vs
> 20k routes and associated processing when the dsl goes up/down/etc.

That's not because it's doing dynamic address assignment - it's
because you're only advertising the aggregate  route from the
BRAS/DSLAM/etc., and you can just as well do the same thing if you're
using static addresses.  You've got pretty much the same sized bunch
of addresses and subnets regardless of how you're assigning them
(except in rare cases where you're getting some statistical gain from
lightly-loaded dynamic address pools), and while static addresses make
it easier to use a dynamic routing protocol instead of static routing,
you don't have to, or you can optionally use a dynamic routing
protocol to hand routing information to the end users and then
summarize at/above your BRAS.

In the ipv4 world, you'd be advertising 1.2.0.0/15 either way, or a
/12 if you're handing out /29s to your users instead of /32s; in the
ipv6 world you'd be advertising 1337::0/39 if you're giving them /56s
or 1337::0/47 if you're giving them /64s.

The real difference is that if they have a static address (and can
therefore advertise it with DNS easily without resorting to
dynamic-DNS trickery), they may start to serve web pages, and then
want to do so reliably, and then start multihoming, and then want a
routing protocol to do better routing, and then either you'll need to
do real work, or else you'll need to tell them to get a real circuit
for their server instead of broadband, or else you'll need to tell
them to use tunnels over the broadband so it's not your DSLAM/BRAS's
problem.
-- 
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             Thanks;     Bill

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