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