nested prefixes in Internet

Tue Sep 27 13:52:10 UTC 2016

* Martin T.:

> 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?

Are the autonomous systems for the /19 and /24 connected directly?  If
they are, (2) is better from a global view (because it needs fewer
routing table entries).  (1) can be better from B's perspective
because it prevents certain routing table optimizations (due to the
lack of the covering prefix).  But (1) can also be worse for B and A's
other customers if /24s (and slightly shorter prefixes) in this part
of the IPv4 address space are commonly filtered.