yet more on distance vectors

[Tony Li <[e-mail]>]

RFC 1267

Appendix 5, section 5.3:

5.3 Processing Update Messages

   In BGP, all UPDATE messages are incremental. Once a particular
   network is listed in an Update message as being reachable through an
   AS path and gateway, that piece of information is expected to be
   retained indefinitely.

   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.

Note that a route which indicates a loop is indeed information.  From
section 9:

   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.

So an implementation could, within the scope of this specification,
keep the route that is advertised that has this loop in the path.  For
example, a multicast implmentation might want to keep this information
as part of its reverse path forwarding database.  An implementation
should never select this path for packet forwarding.

Tony