RFC 9234 lets routers reject route leaks on their own, using BGP Roles and the Only to Customer attribute. We measured who has deployed it, and found two Tier 1 networks unexpectedly stripping OTC.
33.1% of IPv4 Internet paths strip BGP's newest route-leak attribute, even though standard routing protocols explicitly require routers to pass it along. 👇
Route leaks happen when a network takes BGP routes learned from a provider or peer and re-announces them upward or sideways to another provider or peer. Traffic gets misdirected through unintended networks, causing congestion, drops, and severe latency. Historically, preventing these leaks required operators to manually maintain fragile, complex routing filters on every session.
RFC 9234 moves this protection directly into BGP. Cloudflare set out to evaluate how widely this standard is being adopted and whether the broader Internet is honoring its rules.
The standard works by combining session intent with route tagging:
Role negotiation: When an eBGP session comes up, neighbors declare their relationship (Provider, Customer, Peer, RS, or RS-Client). If the roles mismatch—like one side claiming Customer while the other claims Peer—the router rejects the session during the handshake.
Stamping OTC: The first time a route moves sideways or downward, the sending or receiving router attaches an "Only to Customer" (OTC) path attribute containing its Autonomous System Number.
Blocking leaks: Once stamped, an OTC route may only propagate downward to customers. If a router receives an OTC route from a customer, or attempts to send one to a peer or provider, the router identifies the leak and rejects the path.
Because OTC is an optional transitive attribute, core BGP rules require every router along a path to forward it, even if the router does not support RFC 9234 itself. To track adoption, Cloudflare monitored BGP monitoring protocol feeds from its peers and announced test prefixes with OTC attributes attached.
The experiment revealed a surprising twist: 15 networks were actively stripping the attribute. Just two Tier-1 providers—Arelion (AS1299) and GTT (AS3257)—accounted for 96.6% of all stripped IPv4 paths. Both backbones had deployed aggressive attribute-clearing policies years ago as a defensive measure against software crashes caused by malformed BGP updates.
After Cloudflare presented its findings, Arelion updated its network configuration to preserve the OTC attribute across its backbone. Cloudflare identified 67 networks actively setting OTC tags—led largely by Internet Exchange route servers—and has begun deploying RFC 9234 across its own global fleet.
NayaRead shows a summary and links to the original article. The full article is hosted on blog.cloudflare.com.