BGP by default will purge the routes received from a specific peer when a BFD down event occurs and BFD informs BGP about it. The cBit in BFD determines whether BFD is dependent or independent of the Control Plane. Clients like BGP, whose peers are enabled with fast fall over feature with BFD support, can use this BFD cBit support to provide a more deterministic mechanism to do nonstop forwarding (NSF) when BGP graceful restart is enabled along with BFD fast-fallover support for BGP sessions.
When BGP is using BFD for fast fallover feature for remote connectivity detection, BFD can detect some of those failures. If BFD is independent of the control plane, a BFD session failure means that data cannot be forwarded anymore (due to link control failures) and so the BGP Graceful Restart procedures should be aborted to avoid traffic black holes. On the other hand, when BFD is dependent on the control plane, a BFD failure cannot be separated out from the other events taking place in the control plane. When the control plane crashes, a switchover happens and BFD restarts. It is best for the clients (like BGP) to avoid any aborts due to the graceful restart taking place.
The table below describes the handling of BFD down events by BGP.
| Table 1 | BGP handling of BFD Down Event |
| BFD Down Event | Failure - Control Plane Independent ? | BGP Action for NSF (when GR and BFD are enabled) |
|---|---|---|
| BGP control plane detection failure enabled | Yes | Purge Routes |
| BGP control plane detection failure enabled | No | Carry on NSF and keep stale routes in Routing Information Base (RIB) |
| BGP control plane detection failure disabled (the default behavior) | Yes | Purge Routes |
| BGP control plane detection failure disabled (the default behavior) | No | Purge Routes |
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