nested prefixes in Internet

Martin T m4rtntns at
Wed Oct 5 07:45:20 UTC 2016


> Are the autonomous systems for the /19 and /24 connected directly?

Yes they are.

> (1) can be better from B's perspective because it prevents certain routing table optimizations (due to the lack of the covering prefix)

What kind of routing table optimizations are possible if covering /19
prefix is also present in global routing table?

> 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.

Based on my experience /24 is allowed in prefix-filters.. Longer IPv4
prefixes are not.

Roy, Mel:

Could you please elaborate on that option. What kind of advantages
does this have compared to option 2?


On Tue, Sep 27, 2016 at 8:52 PM, Michael Hallgren <mh at> wrote:
> Hi Martin,
> What do you want to do? Move from A to B or add A to B?
> Cheers,
> mh
> Le 27 sept. 2016 17:52, à 17:52, Mel Beckman <mel at> a écrit:
>>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 at> 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
>>>> 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
>>>> 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
>>>> 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
>>>> 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
>>>> behavior I can see with the second option is that if ISP "B" stops
>>>> 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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