"Tactical" /24 announcements

Baldur Norddahl baldur.norddahl at gmail.com
Thu Aug 12 20:30:20 UTC 2021

On Thu, Aug 12, 2021 at 7:39 PM Amir Herzberg <amir.lists at gmail.com> wrote:

> Bill, I beg to respectfully differ, knowing that I'm just a researcher and
> working `for real' like you guys, so pls take no offence.
> I don't think A would be right to filter these packets to; A
> has announced so should route to that (entire) prefix, or A
> is misleading its peers.

You are right that it is wrong but it happens. Some years back I tried a
setup where we wanted to reduce the size of the routing table. We dropped
everything but routes received from peers and added a default to one of our
IP transit providers. This should have been ok because either we had a
route to a peer or the packet would go to someone who had the full routing
table, yes?

So we got complaints. One was a company who would advertise a /20 on a
peering with us. But somewhere else far away they had a site from where
they would announce a /24 from the same prefix. With no internal routing
between the peering site with the /20 to the other site with the /24. We
therefore lost the ability to communicate with that /24.

You see variants of this. For example a large telco has a /16 from which
they many years ago allocated a /24 to a multihomed customer. This customer
left but took their /24 with them. This fact will seldom make the large
telco split up their /16. They will keep it as a /16 but will no longer
route to that /24. The question is also if we really would want a large
telco to explode a large subnet due to this case.


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