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Re: Generally accepted announcement sizes
From: Tony Tauber <ttauber () genuity net>
Date: Thu, 22 Jun 2000 17:35:07 -0400 (EDT)


On Thu, 22 Jun 2000, Mike Heller wrote:

Can anyone point me to a centralized resource for Tier 1 and Tier2
providers'  accept policies?  I have found that when some of my circuits go
down various parts of the 'Net become unreachable and I attributed that to
the size of that announcement being a /24.  I assume that the carriers I'm
having issue with are not using RADB as I registered all of my netblocks,

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).

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

To find out exactly why your multi-homing set-up isn't working, 
I'd work with your providers' operations staff.  Perhaps set up
a time to test the fail over with them on hand to help you analyze
by looking at the routes on both.  It should be possible for them
to help check the behavior of traffic over a third provider as well
if the providers are worth their salt.

Tony





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