nested prefixes in Internet

Victor Sudakov vas at
Sat Nov 19 13:10:05 UTC 2016

Martin T wrote:
> let's assume that there is an ISP "A" operating in Europe region who
> has /19 IPv4 allocation from RIPE. From this /19 they have leased /24
> to ISP "B" who is multi-homed. This means that ISP "B" would like to
> announce this /24 prefix to ISP "A" and also to ISP "C". AFAIK this
> gives two possibilities:
> 1) Deaggregate /19 in ISP "A" network and create "inetnum" and "route"
> objects for all those networks to RIPE database. This means that ISP
> "A" announces around dozen IPv4 prefixes to Internet except this /24
> and ISP "B" announces this specific /24 to Internet.
> 2) ISP "A" continues to announce this /19 to Internet and at the same
> time ISP "B" starts to announce /24 to Internet. As this /24 is
> more-specific than /19, then traffic to hosts in this /24 will end up
> in ISP "B" network.

Excuse me for intruding on American Operators from Siberia, but I find
this topic very interesting. 

I have reports that in case (2), some operators (e.g. Rostelecom)
don't accept the /24 or even /23 prefix on the grounds that it is part
of a larger /19 route already present in the routing table.

Could they have a reason not to accept these more specific
prefixes other than a whim?

Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
sip:sudakov at

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