Re: BGP-4 changes

[Yakov Rekhter <[e-mail]>]

Folks,

How about if we'll use John's text (with some minor editorial
changes) ?

Add to 5.1.4:

  When a route with the MULTI_EXIT_DISC attribute is received over an
  external link it must be possible (based on local configuration) to
  remove or override the value of the attribute. This shall be done
  prior to determining the degree of preference of the route.


Replace 9.1.2.1 with the following:


   In its Adj-RIBs-In a BGP speaker may have several routes to the same
   destination that have the same degree of preference. The local
   speaker can select only one of these routes for inclusion in the
   associated Loc-RIB. The local speaker considers all routes
   with the same degress of preference, both those received from 
   internal peers, and those received from external peers.

   The following tie-breaking procedure assumes that for each candidate
   route all the BGP speakers within an autonomous system can ascertain
   the cost of a path (interior distance) to the address depicted by the
   NEXT_HOP attribute of the route.

   The tie-breaking algorithm begins by considering all equally
   preferable routes and then selects routes to be removed from
   consideration.  The algorithm terminates as soon as only one
   route  remains in consideration.  The criteria must be applied
   in the order specified.

      a) Remove from consideration routes with less-preferred
      MULTI_EXIT_DISC attributes, according to the following
      procedure:

      for m = all routes still under consideration
          for n = all routes still under consideration
              if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
                  remove route m from consideration

      In the pseudo-code above, MED(n) is a function which returns
      the value of route n's MULTI_EXIT_DISC attribute.  If route n
      has no MULTI_EXIT_DISC attribute, the function returns the
      highest possible MULTI_EXIT_DISC value, i.e. 2^32-1.

      Similarly, neighborAS(n) is a function which returns the
      neighbor AS from which the route was received.

      b) Remove from consideration any routes with less-preferred
      interior cost, according to the following procedure.  If the
      BGP speaker has no notion of the cost of a path, skip this
      step (equivalently, consider all routes to have equal costs).

      for m = all routes still under consideration
          for n = all routes in still under consideration
              if (cost(n) is better than cost(m))
                  remove m from consideration

      In the pseudo-code above, cost(n) is a function which returns
      the cost of the path (interior distance) to the address
      depicted by the NEXT_HOP attribute of the route.

      c) If at least one of the candidate routes was received from
      an external peer from a neighboring autonomous system, remove from
      consideration all routes which were received from internal
      peers.

      d) Remove from consideration all routes other than the route
      that was advertised by the BGP speaker whose BGP Identifier
      has the lowest value.


Yakov.