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.engehau...@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 >