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

["John G. Scudder" <[e-mail]>]

At 1:31 AM -0400 8/24/96, Curtis Villamizar wrote:
>Proper MED handling wouldbe loop free.  The current BGP4 RFC allows
>loops to be formed if a prefix is announced from 3 places by 2 AS all
>announcing MED.
>
>MED clearly has to be changed.  The question is which of the two
>proposed fixes to use.  Leaving the current loopfull MED is not
>useful.  We've seen routing loops.

Ah, sorry.  I didn't make it clear that I don't consider any MED handling
which can result in routing loops to be "proper".  I guess I have been
convinced that what Rich calls an "election" is the right way to do MED.
In fact RFC 1771 kind of says this, but in a very vague way:

   The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
   links to discriminate among multiple exit or entry points to the same
   neighboring AS.  The value of the MULTI_EXIT_DISC attribute is a four
   octet unsigned number which is called a metric.  All other factors
   being equal, the exit or entry point with lower metric should be
   preferred.  If received over external links, the MULTI_EXIT_DISC
   attribute may be propagated over internal links to other BGP speakers
   within the same AS.  The MULTI_EXIT_DISC attribute is never
   propagated to other BGP speakers in neighboring AS's.

Note the clauses "same neighboring AS" and "all other factors being equal."

Anyway, I think that we're in violent agreement.

--John

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