Codification of AS 0 Processing
updates
- rfc-4271 — A Border Gateway Protocol 4 (BGP-4)
Extracted elements (8)
AS 0 was already used in RPKI Route Origin Attestations (RFC 6491) to mark prefixes as not to be used in a routing context, requiring BGP implementations to not accept or propagate routes containing AS 0. This document codifies existing behavior already implemented by at least two BGP implementations, correcting an omission in prior BGP specifications.
Authors of future protocol extensions carrying an Autonomous System number are encouraged to recognize that AS 0 is reserved and to provide clear direction on how to handle AS 0 in those extensions.
A BGP speaker MUST NOT originate or propagate a route with an AS number of zero in the AS_PATH, AS4_PATH, AGGREGATOR, or AS4_AGGREGATOR attributes.
A router MUST NOT initiate a BGP connection claiming to be AS 0.
An UPDATE message containing AS number zero in the AS4_PATH or AS4_AGGREGATOR attribute MUST be considered malformed and handled per RFC 6793 error handling procedures.
If a BGP speaker receives AS 0 as the peer AS in an OPEN message, it MUST abort the connection and send a NOTIFICATION with Error Code 'OPEN Message Error' and subcode 'Bad Peer AS'.
The IANA registry for '16-bit Autonomous System Numbers' has been updated so that the entry for AS 0 is simply 'Reserved'.
By ensuring all implementations ignore routes containing AS 0, the document prevents malicious parties from announcing routes with AS 0 to hijack address resources that RPKI ROAs have marked as not in use. Allowing a resource holder to issue a ROA with AS 0 as the only valid origin signals that a prefix should not be routed.