nested prefixes in Internet

Roy r.engehausen at
Tue Sep 27 14:05:18 UTC 2016

Option 3?

ISP A announces the /19 and the /24 while ISP B does just the /24

On 9/27/2016 4:20 AM, Martin T wrote:
> 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

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