3Com S7906E - Switch, Configuration Manual

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How BFD Works
BFD provides a general-purpose, standard, medium- and protocol-independent fast failure detection
mechanism. It can uniformly and quickly detect the failures of the bidirectional forwarding paths
between two routers for protocols, such as routing protocols and Multiprotocol Label Switching (MPLS).
BFD provides no neighbor discovery mechanism. Protocols that BFD services notify BFD of routers to
which it needs to establish sessions. After a session is established, if no BFD control packet is received
from the peer within the negotiated BFD interval, BFD notifies a failure to the protocol, which takes
appropriate measures.
Operation of BFD
Figure 1-1 BFD session establishment (on OSPF routers)
OSPF neighbors
BFD neighbors
Router A Router B
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OSPF advertises the BFD neighbor relationship
BFD session establishment (as shown in the above figure):
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A protocol sends Hello messages to discover neighbors and establish neighborships.
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After establishing neighborships, the protocol notifies BFD of the neighbor information, including
destination and source addresses.
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BFD uses the information to establish BFD sessions.
Figure 1-2 BFD fault detection (on OSPF routers)
BFD fault detection (as shown in the above figure):
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Upon detection of a link failure, BFD clears the session and notifies the protocol of the failure.
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The protocol terminates the neighborship on the link.
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If a backup link is available, the protocol will use it to forward packets.