PATTERN Cited by 1 source
AS-path prepending for traffic engineering¶
AS-path prepending for traffic engineering is the operator pattern of deliberately padding one's own ASN into the outbound AS path multiple times on specific sessions, to bias best-path selection away from that session without fully withdrawing the route.
Mechanism¶
Because receivers prefer shorter AS paths in the BGP best-path
tiebreaker ladder (after local-preference, which is higher
precedence), advertising X, X, X, X, Y on one session and
X, Y on another will push most traffic toward the second
session. The receiver's BGP loop-prevention check still only
checks presence of the receiver's ASN in the path, so the
multiple X repetitions are purely a preference signal — no
real loop is introduced.
When to use it¶
- Backup links: prepend on the backup so traffic only shifts to it when the primary withdraws.
- Outage shaping: quickly push traffic off a circuit that's congested or underperforming, without the hard drop of withdrawal.
- Multi-homed cost management: prefer cheaper transit by making the expensive provider's learned routes less attractive to peers.
Forensic side-effect: heavy prepending on leaked routes¶
signals accident, not attack
See concepts/as-path-prepending for the detailed
forensic read. The short form: attackers want more traffic
on leaked paths (shorter paths, no prepending); accidental
leakers want less traffic (heavy prepending is what they
configure or leave as default on the leg they didn't intend
as primary). In the Cloudflare Venezuela post, nine prepends
of 8048 on leaked paths is the single most direct piece of
evidence against MITM intent.
Cloudflare's stated caveat¶
Cloudflare has argued in Prepends considered harmful that prepending is overused relative to more precise tools (BGP communities, local- preference). Heavy prepending is legitimate when precise tools are unavailable, but it should not be the default traffic-engineering lever.
Seen in¶
- sources/2026-01-08-cloudflare-a-closer-look-at-a-bgp-anomaly-in-venezuela — explicit observation of 9-prepend paths; forensic use as counter-evidence to MITM intent; reference to the Prepends considered harmful framing.