any dangers of filtering every /24 on full internet table to preserve FIB space ?

Matthew Petach mpetach at
Mon Oct 10 22:37:10 UTC 2022

On Mon, Oct 10, 2022 at 8:44 AM Mark Tinka <mark at> wrote:

> On 10/10/22 16:58, Edvinas Kairys wrote:
> > Hello,
> >
> > We're considering to buy some Cisco boxes - NCS-55A1-24H. That box has
> > 24x100G, but only 2.2mln route (FIB) memory entries. In a near future
> > it will be not enough - so we're thinking to deny all /24s to save the
> > memory. What do you think about that approach - I know it could
> > provide some misbehavior. But theoretically every filtered /24 could
> > be routed via smaller prefix /23 /22 /21 or etc. But of course it
> > could be a situation when denied /24 will not be covered by any
> > smaller prefix.
> I wouldn't bank on that.
> I am confident I have seen /24's with no covering route, more so for PI
> space from RIR's that may only be able to allocate a /24 and nothing
> shorter.
> It would be one heck of an experiment, though :-).
> Mark.

I may or may not have done something like this at $PREVIOUS_DAY_JOB.

We (might have) discovered some interesting brokenness on the Internet in
doing so;
in one case, a peer was sending a /20 across exchange peering sessions with
along with some more specific /24s.  After filtering out the /24s, traffic
rightly flowed
to the covering /20.  Peer reached out in an outraged huff; the /24s were
advertised from non-backbone-connected remote sites in their network, that
couldn't fetch content from us anymore.  Traceroutes from our side followed
the /20
back to their "core", and then died.  They explained the /24s were being
from remote sites without backbone connections to the site advertising the
/20, and
we needed to stop sending traffic to the /20, and send it directly to the
/24 instead.
We demurred, and let them know we were correctly following the information
in the
routing table.
They became even more huffy, insisting that we were breaking the internet
by not
following the correct routing for the more-specific /24s which were no
longer present
in our tables.  No amount of trying to explain to them that they should not
an aggregate route if no connectivity to the more specific constituents
existed seemed
to get the point across.  In their eyes, advertising the /24s meant that
everyone should
follow the more specific route to the final destination directly.

So, even seeing a 'covering route' in the table is no guarantee that you
won't create
subtle and not-so-subtle breakage when filtering out more specifics to save
table space.   ^_^;

Having (possibly) done this once in the past, I'd strongly recommend
looking for a
different solution--or at least be willing to arm your front-end response
team with
suitable "No, *you* broke the Internet" asbestos suits before running a git
to push your changes out to all the affected devices in your network.   ;)

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the NANOG mailing list