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RE: Confussion over multi-homing

  • From: Tony Tauber
  • Date: Thu Sep 14 18:12:29 2000

On Thu, 14 Sep 2000, Roeland M.J. Meyer wrote:
> David Lott: Thursday, September 14, 2000 10:34 AM wrote:
> > First, allow me to state the assumptions that I'm under.  I understand
> > the policy to state that if a business needs to multi-home 
> > and requires
> > less space than a /20, then they should request this space from their
> > ISP.  I also understand that there are filters at the /20 
> > boundaries in
> > order to minimize the size of the routing table.

Not *all* boundaries, see below.

> > 
> > Question:  Doesn't this break multi-homing for end users that 
> > need less
> > than a /20?
> Yep, this has been a topic here real resolution. You didn't
> really need to prove the case, it has already been proven.

No.  From a post I made to this list on 6/22/2000:

++> Here's the deal.  If you number out of Provider1's CIDR block
++> but advertise your more-specific to Provider2 and the two Providers
++> touch and Provider1 accepts the more-specific route from Provider2,
++> you should have no problem reaching anyone.
++> Here's the reason: Everyone accepts Provider1's announcement of the block.
++> When your link to P1 is up, any traffic they recieve for your prefix
++> gets routed over that link since they carry your more-specific internally.
++> However, if other providers here the more-specific from P2, they'll
++> send directly via P2 who sends it over the link to you.
++> If your link to P1 goes down, P1 won't see the direct route to you
++> but should see the route via P2 if P1 is accepting it. (Some
++> may either block the announcement or have anti-spoofing packet filters
++> at their borders that block the traffic itself).

As long as Provider1 sees the more-specific from Provider2,
the length is irrelevant.
Does someone disagree?

++> There are many misconceptions about this topic. 
++> Hopefully this explanation has helped someone.