Concept Guide

Table Of Contents
not be received if the link or receiving interface is faulty). The BFD manager noties the routing protocols that are registered with it
(clients) that the forwarding path is down and a link state change is triggered in all protocols.
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 system clears the poll bit and sets the nal bit in its
response. The poll and nal bits are used during the handshake and in Demand mode (refer to BFD Sessions).
140 Bidirectional Forwarding Detection (BFD)