Re: [[e-mail]: Re: BGP4 stuff: Local Preference Computation]

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

At 2:55 PM -0400 9/5/96, Curtis Villamizar wrote:
>The preconfigured policy information is then put into the LOCAL_PREF
>attribute.  This text should be more clear.  It seems that everyone
>who has already implemented BGP4 agrees on the interpretation but the
>text was never very clear.
>
>Don't send LOCAL_PREF to an EBGP peer.  Don't accept LOCAL_PREF from
>an EBGP peer.  Decide what your preference is based on configured
>policy.  Then put that into LOCAL_PREF.  Then use LOCAL_PREF as the
>first value in a comparison.

Speaking of things which aren't clear, the RFC (and draft) list LOCAL_PREF
as a "discretionary" attribute.  It is of course *not* discretionary.  It's
required on IBGP connections and forbidden on EBGP.  The implementor (or
operator) has no discretion as to whether LOCAL_PREF is sent.

This should be fixed by reclassifying LOCAL_PREF as well-known mandatory.

[Other than the text being just plain wrong, the other possibility is that
"discretionary" is being used to mean something like "this is only
mandatory in some cases".  This is a terrible misuse of the language if
true, and should still be fixed, if necessary by inventing some new term
like "well-known sometimes-mandatory" or something.  (Note that the RFC
nowhere defines "discretionary" so we are left with the dictionary
definition, e.g. "left to discretion: exercised at one's own discretion".)]

--John

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