On Juniper ACX/QFX platforms, Bidirectional Forwarding Detection (BFD) sessions behave differently based on the configured timer values and processing mode (Inline, Distributed, or Centralized). When BFD transmit/receive timers are set to 1 second, sessions do not operate in Inline mode on the PFE and instead transition to distributed/centralized processing (handled by the Routing Engine/PPM instead of forwarding hardware).

If Inline remains enabled at this threshold, BFD packets may be missed, leading to session flaps and subsequent BGP resets. Engineering recommends explicitly disabling inline processing when BFD timers are ≥ 1s.

Repeated BGP session flaps between QFX/MX and ACX7K peers after deployment.

Random BFD session down/up events around 1s timer configurations.

Router logs showing BFD down causing BGP Cease (Hard Reset) notifications.

BFD status shows Detect Timer Expiry as the down reason.

Operationally, Singlehop BFD inline is designed for sub-second intervals (qualified minimum ~10ms) and not for ≥1s when inline is enabled

With BFD timers at 1 second, the platform’s BFD implementation does not keep sessions in Inline (PFE) mode. At or above this threshold, the processing path moves away from the OAMP inline hardware to distributed/centralized handling (PPM/RE). If the configuration still attempts inline processing under these conditions, the mismatch can cause missed BFD control packets and session drops, which in turn trigger BGP Cease (Hard Reset) notifications.

Explicitly disable inline processing when BFD timers are ≥ 1s to ensure sessions are handled in centralized/distributed mode and avoid packet misses.

Configuration (recommended for ≥ 1s):

set routing-options ppm no-inline-processing

commit

clear bfd session all

This forces BFD sessions that would otherwise try to use Inline to run in distributed/centralized mode. After committing, clear BFD sessions so they are re-instantiated with the new processing mode.

When to use this fix?

Your BFD minimum-interval is 1,000 ms (1s) or higher.

You observe BGP flaps tied to BFD Down events at 1s timers.

Platforms: Verified behavior on ACX7K families per engineering notes.

Optional verifications:

Check BFD sessions and PPM distribution state:

>show bfd session extensive

>show ppm transmissions protocol bfd detail | match Distributed

>show ppm adjacencies protocol bfd detail | match Distributed

For centralized sessions: Distributed: FALSE on RE.

For distributed sessions: Distributed: TRUE (handled by PFE/ppman).