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Re: Generally accepted announcement sizes
From: Danny McPherson <danny () tcb net>
Date: Thu, 22 Jun 2000 16:07:06 -0600

There are a few points worth reiterating with this model, most
of which Tony has already touched on.

To function properly:

 o This requires that one of your two providers allocate 
   the space and announce the aggregate locally, as well 
   as propagate the more-specific learned from the customer.

 o This requires that the owner of the aggregate accept
   the announcement of the more specific from the second 
   provider as well.  

 o This requires that the customer doesn't have a problem 
   with traffic being routed solely via the primary provider
   when considering peer networks who reject the more-specific
   advertisement from the secondary provider.

 o This requires that the customer doesn't have a problem with
   suboptimal routing to the secondary provider via the primary 
   provider when problems occur, and that the primary provider
   imposes no policies that would disallow this.

 o This model of redundancy also assumes that some type of 
   Byzantine failure with routing internal to the primary 
   providers network is not of concern (hrmm..).
Of course, if any of these are problems, then the whole issues 
becomes, well, slightly more interesting...  


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).

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