Precisely. This is how it's done by providers I've worked with. -mel beckman
On Sep 27, 2016, at 7:06 AM, Roy <r.engehausen@gmail.com> wrote:
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