[c-nsp] BGP routes co-existing with different local-preference

[David Barak]

--- Gert Doering <gert at greenie.muc.de> wrote:

> Hi,
> 
> On Thu, Oct 27, 2005 at 03:29:49PM -0400, Alex
> Rubenstein wrote:
> > Running BGP between /30's or /31's over each
> physical path may not be 
> > 'pretty' or 'sexy', but it is better.
> 
> Besides the fact that it won't do any sort of
> deterministic load balancing 
> that way, unless you have a fairly recent IOS that
> can do BGP multipath.

Why BGP multipath?  Having a single session between
loopbacks means that you only need multiple paths for
a single /32 rather than a full table.  It's a heck of
a lot easier to manage to balance a single prefix than
all of them...

I assume by "deterministic load balancing" you mean
assuring that this /23 uses link 1 and that /20 uses
link 2, right?  

That's pretty darn kludgy - I've only encountered two
types of cases where there are multiple routes between
the same two routers: 

1) equal-size circuits, generally because a single
circuit isn't big enough.  In this case, who needs
determinism?  They're going from point a to point b,
and the properties of the links are equivalent (or you
need a better circuit provider).

2) unequal-size circuits, this is generally a
redundancy mechanism.  This is the power of weighted
static routes - one /32 static route is preferred over
the other.  However, someone should tell this customer
that they'd be even better off having that backup
circuit to a different router entirely (and at that
point having two sessions)

I haven't encountered the customer who has a T3 and a
T1 between the same two routers, and cares about using
the T1 all the time - maybe I've just gotten lucky
that my customers aren't that silly...

So what exactly is the case where having multiple
sessions between the same two routers is an advantage?



David Barak
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