BGP4 on a /20

Security Administrator (or someone claiming to be) wrote...

If all the customers on a multi-billion dollar network could not reach my
network I would think about paying a different upstream provider to peer
with me. I announce a /19 and a /22 to NetRail and UUnet, and they both
do an excellent job of getting Sprint's customers routed to my network.

Marcus R. Williams, Jr.
ISP Programmer / Engineer

Suppose you are the provider with 2 customers with /20's in the same /19.
Both do their own BGP4. You can choose to aggregate and announce the
whole /19 or not. You can choose to pass the /20 announcements or not.

0. Block /20's and don't announce /19.

    In this case, nothing works.

1. Block /20's and announce the whole /19.

    You customers are unable to get routing to work right as this means
    their /20 announcement over their other provider(s) becomes the one(s)
    used, and not your network (but you might think that is good, not to
    put demand on your network).

2. Pass /20's and don't announce /19.

    Your customers cannot get through to providers that block long nets.

3. Pass /20's and announce /19.

    This results in the largest number of routes being added to the tables
    everyone else is keeping.

If providers did NOT do any route filtering based on network length, then
number 2 would not be a problem, and that method could be done. But since
providers do block routes, number 2 has to be discarded and number 3 is

What that means is that when providers block routes longer than /19 they
are causing others to have to make choices that result in more routes
than would otherwise be necessary. Thus, I assert that by doing such
route blocking, they are not achieving the savings in routes they expect
on their own networks, and are causing a greater number of routes for
all the others who are not (yet) filtering.