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RFC 4271, "A Border Gateway Protocol 4 (BGP-4)", January 2006

Source of RFC: idr (rtg)

Errata ID: 5000
Status: Held for Document Update
Type: Technical

Reported By: John Scudder
Date Reported: 2017-04-19
Held for Document Update by: Alvaro Retana
Date Held: 2017-04-27

Section 9.1.1 says:

      If the route is learned from an external peer, then the local BGP
      speaker computes the degree of preference based on preconfigured
      policy information.  If the return value indicates the route is
      ineligible, the route MAY NOT serve as an input to the next phase
      of route selection; otherwise, the return value MUST be used as
      the LOCAL_PREF value in any IBGP readvertisement.

It should say:

      If the route is learned from an external peer, then the local BGP
      speaker computes the degree of preference based on preconfigured
      policy information.  If the return value indicates the route is
      ineligible, the route MUST NOT serve as an input to the next phase
      of route selection; otherwise, the return value MUST be used as
      the LOCAL_PREF value in any IBGP readvertisement.

Notes:

The original text uses "MAY NOT" capitalized as if it were an RFC 2119 keyword. However, RFC 2119 does not have any defined meaning for "MAY NOT". If a reader were to interpret this text as suggesting it is optional -- meaning, in effect, "the route MAY serve as an input to the next phase of route selection" -- that would be wrong and potentially problematic.

The minimal correction would be to use lower-case "may not", which makes the proper meaning reasonably clear. However, the English construct "may not" is notoriously ambiguous, therefore the proposed correction is "MUST NOT".

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After consultation with the idr WG, it was confirmed that the correct interpretation (based on implementation experience) is "MUST NOT". --- Alvaro.

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