nested prefixes in Internet

Roy r.engehausen at
Mon Oct 10 16:04:32 UTC 2016

The solution proposed allows ISP-B to use both paths at the same time, 
needs ISP-C to minimal changes, and has low impact on the global routing 
tables..  I have successfully used it in the past and my old company is 
still using it today.

.On 10/9/2016 11:50 PM, Martin T wrote:
> Florian:
> as I told in my initial e-mail, ISP-B is multi-homed, i.e connected to
> ISP-A(who leases the /24 to ISP-B from their /19 block) and also to
> ISP-C. ISP-B wants to announce this /24 both to ISP-A and ISP-C.
> That's the reason why either solution 1 or 2 in my initial e-mail is
> needed.
> However, I would like to hear from Roy and Mel why do they prefer a
> third option where ISP A announces the /19 and the /24 while ISP B
> does just the /24.
> thanks,
> Martin
> On Wed, Oct 5, 2016 at 11:50 PM, Florian Weimer <fw at> wrote:
>> * Martin T.:
>>> Florian:
>>>> Are the autonomous systems for the /19 and /24 connected directly?
>>> Yes they are.
>> Then deaggregation really isn't necessary at all.
>>>> (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?
>> The /24 prefix could arguably be dropped and ignored for routing
>> decisions.

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