Smaller than a /24 for BGP?
lprehn at mpi-inf.mpg.de
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  and a
While not many ASes redistribute these prefixes, some accept and use
them for their internal routing (e.g., NTT's IPv4 filtering policy ).
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.
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 iecc.com> wrote:
> It appears that Chris J. Ruschmann <chris at scsalaska.net> 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.
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