BGP4 on a /20

Security Administrator security at
Fri Sep 19 14:32:02 UTC 1997

Why should you concern yourself with the problems of a multi-billion
dollar company like Sprint?

Marcus R. Williams, Jr.
security at
ISP Programmer / Engineer

On Thu, 18 Sep 1997, Phil Howard wrote:

> Date: Thu, 18 Sep 1997 18:22:19 -0500
> From: Phil Howard <phil at>
> To: nanog at
> Subject: BGP4 on a /20
> I'm trying to understand what all the implications of running BGP4 on
> a network with a prefix longer than 19 bits.  Here are some of the points
> I am thinking about.
> <flameshields>
> If I go ahead and announce a /20 via two backbones, one of which is
> the provider of the address space, then there will be redundant routes
> for this space as the backbone provider will be announcing the /19
> (or shorter) block themselves.
> If I do this, it adds to the routing table glut, among other things.
> The advantage gained is questionable.  If my link to the provider that
> the space comes from goes down, they are still announcing and I'll only
> be able to reach where my path via the alternate provider is shorter
> than the path to the down provider itself.
> If the provider were to be convinced to stop announcing for my /20,
> then I'm going to get filtered at Sprint and AGIS and whoever else
> is doing this and there won't be any /19 announcement that I can use
> a default path on.
> But the real catch here is that for the provider to stop announcing
> my /20 they have to split their /19 into two /20's.  And if that was
> really a /18 that means they will be announcing a /19 and a /20 where
> before only a /18.  This gets worse the larger their block was.
> Even worse than that, by doing this, they now have a /20 (the other
> half of the /19 my /20 is in) with other customers who will now also
> be filtered out at Sprint and AGIS and whoever else.  While it can be
> OK to me if I want to give up that reachability, this is also imposing
> this on the other customer(s) in the other /20.  So that provider is
> not even likely to do that.
> So, should I add to the glut of routes or should I add to the glut of
> routes?
> This needs to be simpler.
> </flameshields>
> --
> Phil Howard  +-------------------------------------------------------------+
> KA9WGN       | House committee changes freedom bill to privacy invasion !! |
> phil at      | more info:,4,14180,00.html |
> +-------------------------------------------------------------+

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