BGP Unknown Attribute 28 Routing Disruption

In early June 2023, an unusual BGP UPDATE message containing an unknown optional transitive attribute (type code 28) began propagating through the global Internet routing system. Per BGP protocol rules (RFC 4271), optional transitive attributes that a router does not recognize should be passed on to BGP neighbors with the partial bit set. However, some router implementations did not handle this correctly, instead resetting BGP peering sessions upon encountering the attribute.

The cascading effect caused widespread BGP session flapping and routing instability as the malformed or unexpected attribute propagated. Multiple ISPs and network operators reported losing peering sessions and experiencing routing disruptions.

RIPE NCC’s analysis documented the event and noted that implementations differed in how they handle unknown optional transitive attributes: some passed them along with the partial bit set as RFC 4271 requires, others reset the session.