186
echo packets have a format similar to the format of BFD control packets (except that the
Desired Min TX
Interval
and
Required Min RX Interval
fields are null) with UDP port number 3785.
illustrates the
packet format.
Figure 1
BFD packet format
•
Vers: Protocol version. The protocol version is 1.
•
Diag: This bit indicates the reason for the last transition of the local session from
up
to some other
lists the states.
Table 1
Diag bit values
Diag Description
0 No
Diagnostic
1
Control Detection Time Expired
2
Echo Function Failed
3
Neighbor Signaled Session Down
4
Forwarding Plane Reset
5 Path
Down
6 Concatenated
Path
Down
7 Administratively
Down
8
Reverse Concatenated Path Down
9~31
Reserved for future use
•
State (Sta): Current BFD session state. Its value can be 0 for AdminDown, 1 for Down, 2 for Init, and
3 for Up.
•
Poll (P): If set, the transmitting system is requesting verification of connectivity or of a parameter
change. If clear, the transmitting system is not requesting verification.
•
Final (F): If set, the transmitting system is responding to a received BFD control packet that had the
Poll (P) bit set. If clear, the transmitting system is not responding to a Poll.
•
Control Plane Independent(C): If set, the transmitting system's BFD implementation does not share
fate with its control plane
—
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.
•
Authentication Present (A): If set, the Authentication Section is present and the session is to be
authenticated.