A BGP Cease NOTIFICATION Subcode for Bidirectional Forwarding Detection (BFD)
Extracted elements (9)
A BFD session may go Down due to only partial connectivity loss between two BGP speakers, meaning the remote speaker may still be reachable enough to receive the Cease NOTIFICATION. The 'BFD Down' subcode lets the receiver distinguish a BFD-detected forwarding-plane issue from a BGP speaker fault, aiding operational diagnosis.
BGP's minimum Hold Time is at least three seconds (RFC 4271, Sections 4.2 and 4.4), which is too slow for fast failure detection. BFD provides sub-second detection of connectivity loss, and this subcode allows the reason for faster-than-Hold-Timer teardown to be communicated explicitly to the peer.
When RFC 8538 Graceful Restart Hard Reset procedures apply alongside a BFD Down event, the 'BFD Down' subcode must be encapsulated inside the Hard Reset NOTIFICATION data field rather than used as a standalone subcode, ensuring both mechanisms interoperate correctly.
Even when total loss of connectivity prevents the Cease NOTIFICATION message from being delivered, BGP speakers SHOULD still record 'BFD Down' as part of their operational state, for example via bgpPeerLastError (BGP MIB, RFC 4273) or 'last-error' per the BGP YANG model.
When a BGP connection is terminated due to a BFD session going into the Down state, the BGP speaker SHOULD send a NOTIFICATION message with the error code 'Cease' and the error subcode 'BFD Down'.
When the RFC 8538 procedures for sending a Cease/Hard Reset NOTIFICATION are required and the BGP connection is being terminated because BFD went Down, the 'BFD Down' subcode SHOULD be encapsulated in the Hard Reset's data portion of the NOTIFICATION message.
The 'BFD Down' Cease NOTIFICATION subcode is assigned value 10 in the 'BGP Cease NOTIFICATION message subcodes' registry. It is sent to indicate that a BGP connection is being terminated because the associated BFD session transitioned to the Down state.
IANA assigned value 10 with the name 'BFD Down' in the existing 'BGP Cease NOTIFICATION message subcodes' registry, with a reference to this document (RFC 9384).
The 'BFD Down' subcode is purely informational and has no impact on the BGP Finite State Machine beyond what is already documented in RFC 4271, Sections 6.6 and 6.7. Its sole purpose is to help operators correlate network events and diagnose BGP peering issues.