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Re: "Simple" Multi-Homing ? (was Re: CIDR Report)

  • From: Todd Sandor
  • Date: Thu May 18 23:50:59 2000


Since I've been getting a few private email questions on this - here is what I was
what I was able to find out...Basically it can work, but its dependent upon on you
selecting
appropriate ISP providers and that these provider peer  (or I would assume are not
too many ASs away)...
Chris Williams had it correct all along in his mail:
http://www.merit.edu/mail.archives/nanog/msg02315.html

Here is how I understand it:
Yes, the /24s will be filtered out by some [big "meanies" - but give'm lots of $$
and they co-loc for ya - its for
the greater good of the Internet...;-)...].
If Provider A (primay) has a /19 or larger, and you get you /24 from them, then when
your
link to them goes down - your /24 is withdrawn by A but Provider A's /19 or larger
is still globablly visible and packets
will still be sent to Provider A  for your /24. If  provider B is peered with
Provider A, provider B is advertising your /24,
then packets will be routed from A through B to your site.
Note: In this case, Provider A must not filter /24s from his peers to their own
address space.  A must also export
your /24 when your link is up with your AS (in addition to their /19).

See the "BGP Multi-Homing to > 1 ISP Help Required" thread in the
comp.protocols.tcp-ip if you want more info.

Cheers...

Todd Sandor wrote:

>
> This brings me back to my original "operational" question of whether a
> non-aggregated /24 will be globablly visiable when our primary link fails...The
> situation:
>
> "When a customer in one location is using a multi-homed setup to two providers A
> and
> B, with A being the primary (using a /24 from loaned from provider A)
> and B being the secondary (updates via B would have a longer AS_path - using
> default routes with
> local-pref on the primary).  When the customers link to A fails, will the /24
> that
> needs to be now globably visible via B (a non-aggregate IP address for B) NOT be
> globably
> visible because of the BGP filtering policies of some other provider somewhere,
> say C ?
>






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