Smaller than a /24 for BGP?

Lars Prehn lprehn at
Wed Jan 25 05:08:37 UTC 2023

We performed some high-level analyses on these hyper-specific prefixes 
about a year ago and pushed some insights into a blog post [1] and a 
paper [2].

While not many ASes redistribute these prefixes, some accept and use 
them for their internal routing (e.g., NTT's IPv4 filtering policy [3]). 
Rob already pointed out that this is often sufficient for many traffic 
engineering tasks. In the remaining scenarios, announcing a covering /24 
and hyper-specific prefixes may result in some traffic engineering, even 
if the predictability of the routing impact is closer to path prepending 
than usual more-specific announcements. In contrast to John's claim, 
some transit ASes explicitly enabled redistributions of up to /28s for 
their customers upon request (at least, they told us so during interviews).

Accepting and globally redistributing all hyper-specifics increases the 
routing table size by >100K routes (according to what route collectors 
see). There are also about 2-4 de-aggregation events every year in which 
some origin (accidentally) leaks some large number of hyper-specifics to 
its neighbours for a short time.

Best regards,


On 25.01.23 05:12, Forrest Christian (List Account) wrote:
> I have two thoughts in relation to this:
> 1) It's amazing how many threads end up ending in the (correct) 
> summary that making an even minor global change to the way the 
> internet works and/or is configured to enable some potentially useful 
> feature isn't likely to happen.
> 2) I'd really like to be able to tag a BGP announcement with "only use 
> this announcement as an absolute last resort" so I don't have to break 
> my prefixes in half in those cases where I have a backup path that 
> needs to only be used as a last resort.  (Today each prefix I have to 
> do this with results in 3 prefixes in the table where one would do).
> And yes. I know #2 is precluded from actually ever happening because 
> of #1.   The irony is not lost on me.
> On Tue, Jan 24, 2023, 7:54 PM John Levine <johnl at> wrote:
>     It appears that Chris J. Ruschmann <chris at> said:
>     >-=-=-=-=-=-
>     >How do you plan on getting rid of all the filters that don’t
>     accept anything less than a /24?
>     >
>     >In all seriousness If I have these, I’d imagine everyone else
>     does too.
>     Right. Since the Internet has no settlements, there is no way to
>     persuade a network of whom you are not a customer to accept your
>     announcements if they don't want to, and even for the largest
>     networks, that is 99% of the other networks in the world. So no,
>     they're not going to accept your /25 no matter how deeply you believe
>     that they should.
>     I'm kind of surprised that we haven't seen pushback against sloppily
>     disaggregated announcements.  It is my impression that the route table
>     would be appreciably smaller if a few networks combined adjacent a
>     bunch of /24's into larger blocks.
>     R's,
>     John
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