BGP4 on a /20

Bradley Dunn bradley at
Fri Sep 19 17:14:19 UTC 1997

On Thu, 18 Sep 1997, Phil Howard wrote:

> 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.

Ummm...why would your provider be announcing your routes if your link to 
them is down? Sure, they will still be announcing the aggregate that
contains your prefix, but the most specific route always wins (barring
policy that prevents it from winning).

> If the provider were to be convinced to stop announcing for my /20,

Generally a good way to convince your provider to stop announcing your
routes is to stop announcing your routes to them.

> 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.

Providers ALWAYS announce their aggregates. If your provider is flapping
their aggregates based on the state of downstream customers, you need to
identify your provider here so they can be beat on in public.

> 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.

This doesn't make any sense. I think you are forgetting that the most
specific always wins.

> This needs to be simpler.

It is not as complicated as you think. Announce the space that you have
been allocated to all of your providers. Ensure your providers let your
announcements out of their networks.

The only time you may have a problem is if your link to the provider that
allocated you the space goes down. Then you have to hope that that 
provider will allow more specifics of their aggregates to be announced to
them by peers. If they don't allow this, pressure them to. You also have
to hope that the path between the provider that allocated your addresses
and your other provider does not cross a provider that filters at /19.
Ideally the two are peering with each other.

"Seems she thought of me as some mystic, fatalistic, mystical guru
 Me, I haven't got a clue."
	-- Tears for Fears, "Cold"

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