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Re: Verio Peering Question

  • From: Stephen J. Wilcox
  • Date: Sun Sep 30 08:13:19 2001

> > > [2] Want to tune inbound traffic?  Fine... advertise those longer
> > >     prefixes to your upstreams/peers.  But don't make the rest of
> > >     the Internet suffer.  Communities good.  Extra routes bad.
> > 
> > but people dont advertise long prefixes in order to simply make
> > use of two providers for the sake of it, they do it in order to
> IGP-into-BGP causes this, and is hardly for preferring traffic
> from one upstream.
> > create their own unique routing policies which by definition
> > needs to be internet-wide
> Tag a single netblock with a community or MED.  Don't split it
> into two longer prefixes.  Of course, that might require inter-AS
> cooperation.

but if only one provider is agg'ing it will always prefer the most
specific route regardless of tags

> > i would envisage all kinds of problems too where the aggregating
> > upstream accepts your specific routes via another isp by
> > mistake and then your transit traffic ends up going all round
> > the place.. you'd be advertising /24s to peers and all but one
> > transit, with primary transit aggregating up to /16 or
> > whatever, feels bad..
> Hmmmm.  So I have customer X, who also connects to backbone B.
> They advert several blocks, which I agg to  B
> does not agg the blocks... but I'd also agg what I hear from
> B, into the same /19.  No problem here.

no problem for you, but B's other peers wont agg and they will only see
B's more specific path as valid

> Or perhaps a hack... match ^[0-9]*_ and prefer it over longer
> prefixes.  i.e., if you can get there directly, it's better than
> going through another AS.
> Your point definitely merits thought... but I'm not sure that
> it's insurmountable.

i see where youre coming from but its not a part of bgp4. you could write
it into bgp5, trouble is theres a lot of routes out there less specific
and when you turn on this feature it could be very unpredictable.

the other issue is you dont really want to prefer any route, you want them
equally valid and how can you do that unless all the upstreams apply the
same tags/rules 

theres only 20000 AS's, and its the AS that is defined as having a single
routing policy not the prefixes. why cant internet routing be based on AS
announcements, yeah i know you need to rewrite everything but i couldnt
think of a better idea. :)