Den 10/10/2016 kl. 19.24 skrev Niels Bakker:
* r.engehau...@gmail.com (Roy) [Mon 10 Oct 2016, 19:19 CEST]:
I don't think I ever said that ISP-B would announce the /19. That
would only be announced by ISP-A. ISP-B would only announce the /24
that has been delegated to it.
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. You cannot expect 100% of the rest of the
Internet to honour the more specific all the time.
Is that a real problem? In my experience a /24 is honoured almost
universially.
If we assume the big tier 1 transit providers honour the /24
announcement, the only possible way for ISP-A to receive traffic via the
/19 is if ISP-A is directly peered with someone that ignores the /24.
Even if some small amount of traffic does go that route, it might not be
viewed as a problem as the volume is likely to be very low.
Regards,
Baldur
