Superfluous advertisement (was: Friday's Random Comment)

Sat Apr 30 20:05:11 UTC 2016

Simpler, with B and C peered:

   F
  / \
 B---C
  \ /
   A

If B does not send the /24 to F,
then F will send all the traffic to C,
even if A wanted a load balance.

Maybe I could ask the community:
Why do you advertise longer prefixes with the
same nexthop as the shoter prefix?
Is it this use case, or something else?

Thanks,
Jakob.

> -----Original Message-----
> From: Russ White [mailto:7riw77 at gmail.com]
> Sent: Saturday, April 30, 2016 12:35 PM
> To: Jakob Heitz (jheitz) <jheitz at cisco.com>; nanog at nanog.org
> Subject: RE: Superfluous advertisement (was: Friday's Random Comment)
> 
> 
> > A use case for a longer prefix with the same nexthop:
> >
> >    F
> >   / \
> >  D   E
> >  |   |
> >  B   C
> >   \ /
> >    A
> >
> > Suppose A is a customer of B and C.
> 
> This is possible, but only remotely probable. In the real world, D and E are
> likely peered, as are B and C. Further, it's quite possible for F to choose
> the path through E anyway, regardless of A's wishes, or even to load share
> over to the two paths. If it's really a backup path, and you don't want
> traffic on it unless the primary is completely down, then you need to not
> advertise it until you actually need it. One of the various principles of
> packet based routing is that if you advertise reachability, it means
> someone, someplace, might just choose the path you've advertised. You can't
> control what other people choose.
> 
> :-)
> 
> Russ