Bidirectional Forwarding Detection (BFD)

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Hello Premkumar

The BFD control packet has a series of flags within it which are represented by single bits in the header. One of these flags is the C flag, which is the Control Plane Independent (CPI) flag. The use of this flag is further described in RFC 5880 and is described like so:

   Control Plane Independent (C)

      If set, the transmitting system's BFD implementation does not
      share fate with its control plane (in other words, BFD is
      implemented in the forwarding plane and can continue to function
      through disruptions in the control plane).  If clear, the
      transmitting system's BFD implementation shares fate with its
      control plane.

      The use of this bit is application dependent and is outside the
      scope of this specification.  See specific application
      specifications for details.

When it talks about the control plane here, it is primarily talking about the underlying routing protocol that BFD is supporting. So if you’re using OSPF for example, if the OSPF neighborship fails, and the C bit is set, the BFD session will continue to function (if it can) even if the OSPF adjacency has failed. If the C bit is not set, and the OSPF adjacency fails, the BFD session will also go down automatically. Does that make sense??

I hope this has been helpful!

Laz

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