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NOTE: A session state change from Up to Down is the only state change that triggers a link state
change in the routing protocol client.
BFD Packet Format
Control packets are encapsulated in user datagram protocol (UDP) packets. The following illustration
shows the complete encapsulation of a BFD control packet inside an IPv4 packet.
Figure 8. BFD in IPv4 Packet Format
Field Description
Diagnostic Code The reason that the last session failed.
State The current local session state. Refer to BFD Sessions.
Flag A bit that indicates packet function. If the poll bit is set, the receiving system must
respond as soon as possible, without regard to its transmit interval. The responding
Bidirectional Forwarding Detection (BFD)
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