AS_PATH Parser

Parses bare ASNs as AS_SEQUENCE, braces as AS_SET, parentheses as AS_CONFED_SEQUENCE, and square brackets as AS_CONFED_SET. RFC 9774 deprecates AS_SET and AS_CONFED_SET: unless explicitly configured otherwise for a transition, BGP speakers must not advertise UPDATEs containing these segments and must apply treat-as-withdraw when receiving them in AS_PATH or AS4_PATH. This offline parser still accepts and marks the set forms so historical dumps can be inspected; parsing them does not mean they are recommended or normally permitted in modern BGP operation.

For decision-process length, each AS_SEQUENCE member counts once, each AS_SET counts once, and confederation segments do not count outside their confederation. A final AS_SET has multiple possible origin ASNs, so none is selected as uniquely authoritative.

An ASN repeated back to back inside one AS_SEQUENCE or AS_CONFED_SEQUENCE is listed as a prepend candidate, because RFC 4271 and RFC 5065 let a speaker prepend its own AS number more than once. Other repeats (separate positions, AS_SET members, or across segments) are listed separately. A repeat across segments is not classified as a prepend candidate automatically, but a speaker starts a new segment when an AS_SEQUENCE or AS_CONFED_SEQUENCE would exceed 255 ASNs (RFC 4271 §5.1.2, RFC 5065 §4.1), so it may still have been prepended.

A repeated ASN alone does not show an AS loop, and a prepended path is still an AS loop for a receiver whose own ASN is in it: AS loop detection compares the path with the receiving router's own ASN, so it is evaluated only when you enter a local ASN. The number alone does not say which role it has at the receiver, so each role is shown as a conditional result. As the AS number of a speaker outside any confederation (RFC 4271 §9.1.2), a match in AS_SEQUENCE or AS_SET is a loop; such a speaker must treat confederation segments as a malformed AS_PATH (RFC 5065 §5), so that check is not evaluated when they are present. As an AS Confederation Identifier, a match anywhere in the AS_PATH is a loop; as a Member-AS Number, a match in AS_CONFED_SEQUENCE or AS_CONFED_SET is a loop (RFC 5065 §4).

This is offline structural inspection, not a guarantee of how a router accepts or discards the route; implementation settings that accept the own ASN, such as allowas-in, are not modeled. AS_TRANS (AS23456) can stand for a four-octet ASN (RFC 6793 §4.2.2); this tool does not reconstruct the path from AS4_PATH (RFC 6793 §4.2.3), so with AS_TRANS present the loop check covers only the entered AS_PATH and a missing match is not conclusive.