nested prefixes in Internet

Owen DeLong owen at
Mon Oct 10 20:27:38 UTC 2016

> On Oct 10, 2016, at 12:44 PM, Baldur Norddahl <baldur.norddahl at> wrote:
> Den 10/10/2016 kl. 19.24 skrev Niels Bakker:
>> * r.engehausen at (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 universally.

In my experience, with notable exceptions, ISPs don’t like to provide transit to people who aren’t paying them, so if it becomes enough traffic to get noticed, it’s not at all unlikely that ISP-A would start dropping it, even if they didn’t ignore the prefix.

> 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.

Not true… There are myriad reasons that the /24 might not reach a network peered with ISP-A, including the possibility of being a downstream customer of a network peered with or buying transit from ISP-A. In the latter case, not an issue, since it’s paid transit, but in the former (peered, not transit), again, ISP-A is probably not super excited to carry traffic that someone isn’t paying them to carry.

> 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.

Until some clever miscreant notices the situation and decides to exploit it for a dDOS.


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