[c-nsp] Re: URPF on small BGP-enabled customers?

[David J. Hughes]

On 04/06/2005, at 6:35 AM, Joe Maimon wrote:

> And if they were mutlihomed to the same two peers that you announced
> no-export?
>
> Should they still be taking default then?
>
> IMHO all those who tag no-export and then are surprised when people
> surprise surprise dont get those routes, those are the daft ones.

Perhaps you should consider the possible scenarios before making such 
assumptions.  We present our prefixes to 6 upstream ASes (every tier-1 
in the country plus some others).  We tagged no-export to one of our 
upstreams for our own traffic engineering purposes.

The "daft" provider to which I referred was single homed, but still ran 
BGP and neither generated a default internally nor accepted one from 
their upstream.  It appears that they were not getting a full table 
from their provider (or were filtering it) as that would have included 
our prefixes via 5 indirect paths.


> If a router gets a full BGP table from its peer it should never need a
> default route.

The assumption that getting a "full table" from only a single provider 
will ensure you get every prefix on the public network is fundamentally 
flawed.  By definition you are getting THEIR idea of a full table, 
regardless the filtering etc that their peering policies may impose.  
As a customer you have no control over their peering policies. I 
certainly wouldn't run without a default in that environment.


David
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