BFD Packet

This section describes the format of the BFD packet.

BFD packets fall into two types, namely, BFD control packet and BFD echo packet.

BFD Control Packet

BFD control packets are encapsulated in UDP packets for transmission, and the destination port of UDP is port 3784.

A BFD control packet consists of a mandatory part and an optional authentication part. Figure 1 shows the format of the BFD control packet.

Figure 1 Format of the BFD control packet

FW does not support the BFD function.

Table 1 shows the description of each field in the packet.

**Table 1** Description of each field in the BFD control packet
Field	Length	Description
Vers (Version)	3 bits	Indicates the version number of the protocol. The current version number is 1.
Diag (Diagnostic)	5 bits	Indicates the cause that the status of the latest session changes from Up to other status in the local system. Different values indicate different causes: 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 to 31: Reserved for future use
Sta (State)	2 bits	Indicates the status of the current BFD session. Different values indicate different statuses: 0: AdminDown. Indicates that the BFD session is in administrative Down state. 1: Down. Indicates that the BFD session is Down or just established. 2: Init. Indicates that the BFD session can communicate with the peer end and the local end expects the session to enter the Up state. 3: Up. Indicates that the BFD session is successfully established.
P (Poll)	1 bit	Indicates the bit for connection request confirmation. Different values indicate different meanings: 1: indicates that the sending system requests the confirmation of the connection or the parameter changes. 0: indicates that the sending system does not request the confirmation of the connection or the parameter changes.
F (Final)	1 bit	Indicates the bit determining whether the sending system responds to a BFD control packet with P bit as 1. Different values indicate different meanings: 1: indicates that the sending system responds to a BFD control packet with P bit as 1. 0: indicates that the sending system does not respond to a BFD control packet with P bit as 1.
C (Control Plane Independent)	1 bit	Indicates the bit determining whether BFD control packets are transmitted on the control plane. Different values indicate different meanings: 1: indicates that the sending system implements BFD independent of the control plane. That is, BFD packets are transmitted on the forwarding plane. BFD continues to work even if the control plane fails. 0: indicates that BFD packets are transmitted on the control plane.
A (Authentication Present)	1 bit	Indicates the bit determining whether BFD control packets contain the authentication field. Different values indicate different meanings: 1: indicates that the BFD control packet contains the authentication field, and the session needs to be authenticated. 0: indicates that the BFD control packet does not contain the authentication field, or the session does not need to be authenticated. NOTE: FW does not provide the BFD authentication function currently. The A bit is set to 0 all the time.
D (Demand)	1 bit	Indicates the demand mode operation bit. Different values indicate different meanings: 1: indicates that the sending system expects to run in demand mode. 0: indicates that the sending system does not expect to or cannot run in demand mode.
R (Reserved)	1 bit	This field is set to 0 when a BFD control packet is sent. This field is ignored when a BFD control packet is received.
Detect Mult (Detect time multiplier)	1 byte	Indicates the detection time multiplier, that is, the maximum number of continuous loss of packets permitted by the packet receiver. The bit is used to check whether the link is normal. Demand mode: uses the local detection time multiplier. Asynchronous mode: uses the detection time multiplier of the peer end.
Length	1 byte	Indicates the length of a BFD control packet, in bytes.
My Discriminator	4 bytes	Indicates a unique non-zero discriminator value generated by the sending system. The value is used to differentiate multiple BFD sessions of a system.
Your Discriminator	4 bytes	Indicates the value of My Discriminator sent from the remote system. If this value is not received, the field is set to 0.
Desired Min Tx Interval	4 bytes	Indicates the desired minimum interval for sending BFD control packets by the local system, in microseconds.
Required Min Rx Interval	4 bytes	Indicates the minimum interval required between receiving two BFD control packets, in microseconds.
Required Min Echo Rx Interval	4 bytes	Indicates the minimum interval required between receiving two BFD echo packets, in microseconds. If the interval is set to 0, the sending system cannot receive BFD echo packets.
Auth Type	1 byte	Indicates the authentication type of BFD control packets. Different values indicate different authentication types: 0: Reserved 1: Simple Password 2: Keyed MD5 3: Meticulous Keyed MD5 4: Keyed SHA1 5: Meticulous Keyed SHA1 6 to 255: Reserved for future use
Auth Len	1 byte	Indicates the length of the authentication field, including the authentication type field and the authentication length field, in bytes.
Authentication Data	2 bytes	Indicates the authentication data.

BFD Echo Packet

BFD echo packets provide a fault detection mechanism independent from BFD control packets. The local end sends and receives the packets. The peer end returns the packets over the reverse channel without processing the packets. Therefore, the format of BFD echo packet is not defined by the BFD protocol. The only requirement is that the sender can distinguish sessions according to packet contents.

BFD echo packets are encapsulated in UDP packets for transmission. The destination port number is 3784. The destination IP address is the address of the sending interface. The source IP address is specified manually.