Re: Addr: Re: BGP-4 - revised I-D

[Tony Li <[e-mail]>]

   > - group the routes by neighboring AS
   > - within each group, select the routes with the best MED
   > 
   > which is also what I think is the intended interpretation.  Alternate
   > interpretations include:
   > 
   > i)
   > - group the routes by neighboring AS
   > - within each group, select _a_ route with the best MED

   Interpretation i) is the same as the first.  

No, it's not.  If you follow i) verbatim, then you must select one and only
one route.  As we have not specified which one, it's presumably left up to
the implementor.  You and I both know that this is disaster.  ;-)

   If there are two routes
   with the same AS and the same MED, the next criteria is IGP cost or
   advertising router IP address.  Whether you bring one or both into the
   next comparison, you end up with the same decision whether you
   consider MED and they tie in the first comparison or you ignore MED in
   the second comparison.

Did you really mean to say this?  Suppose I have routes A, B, C, ... and
that A & B are the same MED.  Without loss of generality, suppose that A
has a better IGP metric.  

If we bring A into the next comparison, then it will be selected.  If we do
NOT bring A into the next comparison, then B may be selected.  Loop.

Tony