Network & Infrastructure / BGP Policy & Internet Edge
AS-path prepending cannot guarantee a chosen inbound path
A provider continues delivering traffic over the supposedly less-preferred circuit after the enterprise prepends its AS. The remote network may prioritize local policy above AS-path length.
Environment and inputs: BGP speakers and route reflectors; policy syntax, RPKI integration and address-family behavior depend on the implementation. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Collect received advertisements from approved looking glasses, provider communities and interface traffic by destination group. Separate inbound observations from the enterprise's outbound routing decisions.
Explain the result
Prepending influences one attribute visible to remote networks. It cannot override their local preference, and different networks can select different paths for the same prefix.
Plan the fix
Use documented provider traffic-engineering communities where available and negotiate routing intent with the provider. Keep announcements consistent with authorized prefix ownership and failover needs.
Check the result and rollback
Compare traffic over a representative business interval and verify failover without withdrawing both paths. Restore the previous export policy if latency, concentration or unexpected route propagation worsens.