shim6 @ NANOG (forwarded note from John Payne)

Todd Vierling tv at
Tue Feb 28 15:34:44 UTC 2006

On Tue, 28 Feb 2006, Iljitsch van Beijnum wrote:

>    A --- B
>  /         \
> X             Y
>  \         /
>    C --- D
> C's link to D may be low capacity or expensive, so D would prefer it if X
> would send traffic to Y over another route if possible. C can make this happen
> in BGP by prepending its AS one or more times so X will see the following AS
> paths:
> A B Y
> C C C D Y
> All else being equal, X will choose the path over A to reach Y.

There's plenty of route mangler technologies out there that provide
overriding BGP information to borders that trumps path length.  "All else"
is often not as equal as you seem to expect.

It's time to wake up and smell the intelligent routing trend.  The
usefulness of prepending is rapidly dwindling.  Don't try to push it as a
future-compatible solution; it is not.  Prepending is not a tool; it is a
hack that has outlived its usefulness.

> Another capability that would be hard to replicate with shim6 is selective
> announcement.

Now, selective announcement is something completely different -- but it's
still a historical hack for lack of better mechanisms in BGP[34].  If the
route isn't there at all, it won't be selected in today's world.  But also
consider this:

- C does not advertise the prefix for Y, but it does have the next
  superprefix for Y (and C is "transit", so the superprefix must be
  considered valid);

- X's link to A dies.

So X will still try to push packets over C to reach Y, and per the existence
of the superprefix on C, that route should[!] be valid.

Don't think this will forever be a rare circumstance, either.  The route
mangling technologies I mentioned above are now starting to offer the
ability for traffic to go out a "transit" neighbor so long as some
containing prefix is advertised (even if it's not the most specific).

Traffic engineering is happening on both ends of the BGP mesh *today*, so
you should present any proposed solution in that context.

-- Todd Vierling <tv at> <tv at> <todd at>

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