nested prefixes in Internet

Tue Sep 27 11:20:04 UTC 2016

Hi,

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.

Which approach is better? To me the second one seems to be better
because it keeps the IPv4 routing-table smaller and requires ISP "A"
to make no deaggregation related configuration changes. Only bit weird
behavior I can see with the second option is that if ISP "B" stops for
some reason announcing this /24 network to Internet, then traffic to
hosts in this /24 gets to ISP "A" network and is blackholed there.

thanks,
Martin