Re: URPF on small BGP-enabled customers?

["Patrick W. Gilmore" <patrick () ianai net>]

On Jun 3, 2005, at 9:30 AM, christian.macnevin () uk bnpparibas com wrote:

> At an old transit provider I was at, we had a pig of a time dealing  
> with
> uRPF. It doesn't like asymmetric routing at all, which is  
> commonplace when
> you've got customers homed at exchange points for one.
>
> I imagine the simplest and most foolproof way around directly  
> connected
> providers blackholing your traffic is announcing more specific  
> prefixes
> down the one you're currently favourint, and just the aggregates  
> for same
> into the second. Good luck if you've only got a bunch of non- 
> contiguous
> /24s..

<disclaimer> Not uRPG guru </disclaimer>

Why would that work?  If I see a /16 from my customer and a /19 from  
a peer, I will still pick the /19, and strict uRPF should drop any  
packets from that /19 coming the customer interface, right?

Not to mention the Really Bad Things associated with deaggregation.

Perhaps a simpler way is to announce your entire allocation and put  
no-export on things you want to come in your other provider?  ^1239$  
will still pick those routes, but no one else will see them.   
Although sprint is a _VERY_ large network when you include  
downstreams, their own AS is rather tiny compared to the whole Internet.

--
TTFN,
patrick