Re: Confussion over multi-homing

[Patrick Evans <pre () pre org>]

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                     www.pre.org/pre