BFD for OSPF Session Creation and
Deletion
All Open Shortest Path First (OSPF) neighbors
are associated with an interface. When BFD is enabled on an interface and at the global
level, BFD sessions are created for all OSPF neighbors that are in greater than the
2-Way state.
Each interface can have its own BFD timers. OSPF
does not influence the BFD timer value for the session. The BFD module uses the
configured BFD value or the default value when creating the session. A single-hop
session is created for OSPF neighbors.
Note
- When BFD for an OSPF session is configured, the normal OSPF hello mechanism is
not disabled.
- OSPF neighbor sessions do not flap when BFD is enabled or disabled on the
interface where the OSPF session is associated.
- OSPF assumes full connectivity between all systems on multi-access media such as
LANs. If BFD is running on only a subset of systems on such a network, the
assumptions of the control protocol may be violated, with unpredictable
results.
BFD for OSPF sessions are deleted in the
following scenarios.
- An OSPF neighbor session moves to a state below
2-Way. OSPF triggers a BFD session deletion setting the reason as Path Down.
When BFD receives this notification, it deletes the corresponding session. The
remote BFD neighbor detects this as a detection timer expiration and propagates this
session down to OSPF to terminate the session.
- OSPF is disabled on an interface. All BFD
sessions associated with each neighbor are deleted.
- BFD is administratively disabled on an
interface. BFD reports this event as a port change notification to OSPF.
Upon receipt of this notification, a BFD session deletion for each neighbor is sent
and BFD removes these sessions if no other client is using the same sessions. BFD
communicates this change to the remote peer. When the remote BFD peer receives this
Remote Admin Down event, it propagates this event to the OSPF protocol. OSPF does
not provide any action for this event.
