nested prefixes in Internet

Niels Bakker niels=nanog at
Mon Oct 10 20:39:18 UTC 2016

* baldur.norddahl at (Baldur Norddahl) [Mon 10 Oct 2016, 21:45 CEST]:
>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 
>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 
>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.

Sure, continue believing that, until you run into a customer who 
takes a /24 and then announces it no-export just to your upstream, 
and mysteriously has lots of outages on their link to you.

Aside from that, this does happen and apparently much more often 
than you think.  Think CDN nodes with partial views.  There are 
plenty networks that filter on or close to RIR boundaries due to 
hardware limitations as well.

	-- Niels.

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