This article provides information about the behavior of BFD, when the interface is brought down via CLI or by pulling the cable out.

• BFD never has any interface information with it.
• BFD depends on RPD to provide, create, and destroy sessions.
• So, the behavior of BFD varies when the interface is brought down via CLI or by pulling the cable out.

• When interface is disabled by CLI :
  
  When the interface is disabled by CLI, the kernel provides the interface down information to RPD. RPD removes the BFD sessions and BFD, in turn, sends out an ADMIN DOWN to the peer. If the interface is not physically down yet, then the ADMIN DOWN packet will reach the peer and it will send ADMIN DOWN to RPD.
  
  As per the standard, RPD should not act on ADMIN DOWN; so OSPF is not brought down. When the interface is brought down via CLI, it takes a few milliseconds to bring down the interface. In case of the GRE tunnel being disabled, the interface was actually down after a few seconds.
• When interface goes down on its own :
  
  The physical interface is already down and the kernel provides the interface down information to RPD. RPD removes the BFD sessions and BFD, in turn, sends out ADMIN DOWN to the peer. This packet will never reach the peer, as the interface is already down. At the other end, BFD will not receive packets for detect time and eventually, BFD times out and sends DOWN to RPD. RPD will act on DOWN and bring down OSPF sessions.