3Com S7906E Configuration Manual page 2242

No detection time resolution is defined in the BFD draft. At present, most devices supporting BFD
provide detection measured in milliseconds.
BFD detection methods
Single-hop detection: Detects the IP connectivity between two directly connected systems. Single
hop means one hop for IP forwarding.
Multi-hop detection: Detects any of the paths between two systems. These paths may have
multiple hops and maybe overlapped.
Bidirectional detection: Sends detection packets at two sides of a bidirectional link to detect the
bidirectional link status, thus to implement a link fault detection rate in milliseconds. (BFD LSP
detection is a special case, in which BFD control packets are sent in one direction, and the peer
device reports the link status through other links.)
BFD session modes
Control packet mode: Both ends of the link exchange BFD control packets to monitor link status.
Echo mode: One end of the link sends Echo packets to the other end, which then forwards the
packets back to the originating end, thereby monitoring link status in both directions.
BFD operating modes
Before a BFD session is established, there are two BFD operating modes: active and passive.
Active mode: In this mode, BFD actively sends BFD control packets regardless of whether any
BFD control packet is received from the peer.
Passive mode: In this mode, BFD does not send control packets until a BFD control packet is
received from the peer.
At least one end must operate in the active mode for a BFD session to be established.
After a BFD session is established, both ends must operate in one of the following two BFD operating
modes: asynchronous and command.
Asynchronous mode: A device operating in the asynchronous mode periodically sends BFD control
packets. The peer considers that the BFD session is down if it receives no BFD control packet
within the BFD interval.
Demand mode: In this mode, it is assumed that a system has an independent way of verifying the
connectivity to the peer system. Once a BFD session is established, such a system may stop
sending BFD control packets, except when the system feels the need to verify connectivity
explicitly.
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