Re: [babel] Alvaro Retana's Discuss on draft-ietf-babel-rfc6126bis-12: (with DISCUSS and COMMENT)

[Juliusz Chroboczek <[e-mail]>]

Would adding the following informative appendix solve the problem?  Are
you going to complain that it's not referenced in the document body?

Appendix C.  Route filtering

   Route filtering is a procedure where an instance of a routing
   protocol either discards some of the routes announced by its
   neighbours or learns them with a metric that is higher than what
   would be expected.  Like all distance-vector protocols, Babel has the
   ability to apply arbitrary filtering to the routes it learns, and
   implementations of Babel that apply different sets of filtering rules
   will interoperate without causing routing loops.  The protocol's
   ability to perform route filtering is a consequence of the
   definitions in Section 3.5.2: Babel can use any metric that is
   strictly monotonic, including one that assigns an infinite metric to
   a selected subset of routes.  (See also Section 3.8.1, where requests
   for nonexistent routes are treated in the same way as requests for
   routes with infinite metric.)

   It is not in general correct to learn a route with a metric smaller
   than the one it was announced with, or to replace a route's
   destination prefix with a longer one.  Doing either of those may
   cause persistent routing loops.

   Route filtering is a useful tool, since it allows fine-grained tuning
   of the routing decisions made by the routing protocol.  Accordingly,
   some implementations of Babel implement a rich configuration language
   for configuring route filtering.  At a minimum, these implementations
   allow selecting routes for filtering by incoming interface and by
   destination prefix.

   In order to limit the consequences of misconfiguration, Babel
   implementations provide a reasonable set of default filtering rules
   even when th don't allow configuration of filtering by the user.  At
   a minimum, they discard routes with a destination prefix in
   fe80::/64, ff00::/8, 127.0.0.1/32, 0.0.0.0/32 and 224.0.0.0/8.