On Mon, Oct 10, 2016 at 12:24 PM, Niels Bakker <niels=na...@bakker.net> wrote:
> * r.engehau...@gmail.com (Roy) [Mon 10 Oct 2016, 19:19 CEST]:  [snip]
>> If the ISP-A/ISP-B link goes down then the /24 would be seen only via
>> ISP-C which is the desired result.
>
> What if ISP-A then receives traffic inside its /19 destined for ISP-B's /24?
> It will have to send it over transit and won't bill ISP-B for that traffic.

This is still the favored approach over ISP-A  de-aggregating their
address space to
support some multi-homed customers.

It's all something that ISP-A can put in their contract with ISP-B.
A can bill ISP-B  per  packet-byte routed  with destination of B's
/24,  if they wanted,
or reach other mutually agreeable conditions that make sure the /24 carve out
does not commit A to giving service to B involving more traffic than B pays for.

Or they can list Rate #1  for incoming traffic to B's /24 sent to a
peering link with ISP-B,
and  apply pricing Rate #2  for  packets destined for B's  /24
between two transit providers.

...However,  ISP-A and ISP-B  should have a link;  Either a physical connection
or a virtual one (such as a tunnel), with BGP peering between A and B,
 for  ISP-B to be
using a /24 from A's IP space  with other providers.


> You cannot expect 100% of the rest of the Internet to honour the more
> specific all the time.

That is true, but not really notable / isn't really a significant problem.

Probably close enough to 100% of the internet to honor it 99% of the time,
and the 1% that don't  probably  will likely not be one of  ISP A or
B's adjacent providers.
(If they are, then A or B will open a ticket,  and the /24 will be
fixed to work as desired)

Heck.... on some occasion when ISP A has an outage,  the /19 won't be there,
so  less than 100% of the internet might 'honor'  that one at times, as well.

--
-JH

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