RFC 1267

[Tony Li <[e-mail]>]

   Thanks, I now understand what you are talking about.  However, RFC
   1267 does not indiciate the order in which the following should be
   applied:

   (1) Generally speaking, the rules for comparing routes among several
       alternatives are outside the scope of this document.  There are two
       exceptions:

       - If the local AS appears in the AS path of the new route being
	 considered, then that new route cannot be viewed as better than
	 any other route.  If such a route were ever used, a routing loop
	 would result.

   (2) In order for a route to a network to be removed, it must be
       explicitly listed in an Update message as being unreachable or with
       new routing information to replace the old. Note that a BGP peer will
       only advertise one route to a given network, so any announcement of
       that network by a particular peer replaces any previous information
       about that network received from the same peer.

Fixed.  See BGP-4.  When an update is received, the old path is first
removed.  Then the new path may be examined.

   The order of application definitely matters so that the routes that
   will cause the looping are actually removed.  However, all of this is
   predicated on the fact the route .. Y .. X ..  Z is advertised back to
   X and the route .. X .. Y .. Z is advertised back to Y.  If these
   advertisements are not made, then the looped routes X .. Y .. Z and
   Y .. X .. Z will not be removed.

Indeed.  You must always advertise your paths to all of your
neighbors.  And if administrative filtering would prevent you from
advertising the path, you must send an unreachable.

Tony