HP 4800G Series Configuration Manual page 1588

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. It tears down the BFD session if it receives no BFD control packet from the peer within the
BFD interval.
Demand mode: In this mode, it is assumed that a system has an independent way of verifying that
it has connectivity to the other system. Once a BFD session is established, such a system may ask
the other system to stop sending BFD Control packets, except when the system feels the need to
verify connectivity explicitly, in which case a short sequence of BFD Control packets is exchanged,
and then the far system quiesces. Demand mode may operate independently in each direction, or
simultaneously.
At present, only the asynchronous mode is supported.
When a BFD session operates in Echo mode, the session is independent of the operating mode.
When the connectivity to another system needs to be verified explicitly, a system sends several
BFD control packets that have the Poll (P) bit set at the negotiated transmit interval. If no response
is received within the detection interval, the session is considered down. If the connectivity is found
to be up, no more BFD control packets are sent until the next command is issued.
Dynamic BFD parameter changes
After a BFD session is established, both ends can negotiate the related BFD parameters, such as the
minimum transmit interval, minimum receive interval, initialization mode, and packet authentication
mode. After that, both ends use the negotiated parameters, without affecting the current session state.
Authentication modes
BFD provides the following authentication methods:
Simple: Plain text authentication
MD5: MD5 (Message Digest 5) authentication
SHA1: SHA1 (Secure Hash Algorithm 1) authentication
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