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Re: Confussion over multi-homing

  • From: John Fraizer
  • Date: Fri Sep 15 06:14:31 2000

On Fri, 15 Sep 2000, Patrick Evans wrote:

> On Thu, 14 Sep 2000, John Fraizer wrote:
> > They won't see it even if their /20 announcement is being filtered
> > as long as their upstream is listening to it.  (Upstream being the
> > carrier they obtained the /20 from.)  This is because even though
> > some ISP/NSPs may not listen to the specific /20 announcement, I
> > haven't found any who will ignore a /19 or /17 announcement for
> > the master block.  As long as they know how to get to that block,
> > and teh "upstream" knows how to get to the /20, the route
> > filtering isn't going to be apparent at all... Until the time that
> > the "upstream" has connectivity issues.
> > 
> So the general picture is...
> I advertise a /22 to ISPs A and B, my upstream providers, using space
> allocated from provider A's assignment.
> ISP A's connection to the world (okay, ISP A might not exactly be
> UUNet) goes tits up at which point the world ceases to see any
> announcements because the aggregate with the shorter prefix has
> vanished, and they're filtering the longer prefix announced via ISP B.
> The whole thing seems to be dependent on ISP A not having problems
> connecting to external ASes.
> Or am I missing something big? If multi-homing still leaves me
> dependent on one of my upstreams being alive, there would appear to be
> something very wrong...
> -- 
> Patrick Evans - Sysadmin, bran addict and couch potato
> pre at pre dot org           

That's where you stand if people are filtering your /22
announcement.  You're dependent on less specific announcement making it
through.  Keep in mind that you are only dependent on ISP A for traffic to
providers who are filtering on the /20 boundry.  There are LOTS of folks
out there that filter on the /24 boundry that your announcement will make
it through to via ISP B.

John Fraizer
EnterZone, Inc

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