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174 | Bidirectional Forwarding Detection
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handshake. At this point, the discriminator values have been exchanged, and the transmit intervals
have been negotiated.
4. The passive system receives the control packet, changes its state to Up. Both systems agree that a
session has been established. However, since both members must send a control packet—that requires
a response—anytime there is a state change or change in a session parameter, the passive system sends
a final response indicating the state change. After this, periodic control packets are exchanged.
Figure 9-2. BFD Three-way Handshake
Session state changes
Figure 9-3 shows how the session state on a system changes based on the status notification it receives
from the remote system. For example, if a session on a system is down, and it receives a Down status
notification from the remote system, the session state on the local system changes to Init.
ACTIVE System
PASSIVE System
Steady Rate of Control Packets
Version: 1
Diag Code: 0 (assumes no previous session)
State: Down
Flag: P:1
Detect Multiplier: User-configurable
My Discriminator: X ( Active System Session ID)
Your Discriminator: 0
Desired Min TX Interval: User-configurable
Required Min RX Interval: User-configurable
Required Min Echo RX Interval: User-configurable
Init State Change
Version: 1
Diag Code: 0 (assumes no previous session)
State: Init
Flag: F: 1
Detect Multiplier: User-configurable
My Discriminator: Y (Passsive System Session ID
)
Your Discriminator: X
Desired Min TX Interval: User-configurable
Required Min RX Interval: User-configurable
Required Min Echo RX Interval: User-configurab
le
Transmit Interval: User-configurable
Up State Change
V
ersion: 1
D
iag Code: 0 (assumes no previous session)
S
tate: Up
F
lag: P: 1
D
etect Multiplier: User-configurable
M
y Discriminator: X
Y
our Discriminator: Y
D
esired Min TX Interval: User-configurable
R
equired Min RX Interval: User-configurable
R
equired Min Echo RX Interval: User-configurable
Version: 1
Diag Code: 0 (assumes no previous session)
State: Up
Flag: F: 1
Detect Multiplier: User-configurable
My Discriminator: Y
Your Discriminator: X
Desired Min TX Interval: User-configurable
Required Min RX Interval: User-configurable
Required Min Echo RX Interval: User-configurab
le
Up State Change
Default Session State: Down
Default Session State: Down
Periodic Control Packet
V
ersion: 1
D
iag Code: 0 (assumes no previous session)
S
tate: Up
F
lag: P: Clear
D
etect Multiplier: User-configurable
M
y Discriminator: X
Y
our Discriminator: Y
D
esired Min TX Interval: User-configurable
R
equired Min RX Interval: User-configurable
Required Min Echo RX Interval: User-configurable
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