nested prefixes in Internet

Baldur Norddahl baldur.norddahl at
Mon Oct 10 19:44:55 UTC 2016

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 /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.



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