On Mon, Oct 10, 2016 at 12:24 PM, Niels Bakker <niels=nanog@bakker.net> wrote:
* r.engehausen@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