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BFD detection methods
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Single-hop detection: Detects the IP connectivity between two directly connected systems.
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Multi-hop detection: Detects any of the paths between two systems. These paths have multiple hops
and may be overlapped.
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Bidirectional detection: Sends detection packets at two sides of a bidirectional link to detect the
bidirectional link status, finding link failures 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
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Control packet mode: Both ends of the link exchange BFD control packets to monitor link status.
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Echo mode: One end of the link sends Echo packets to the other end, which then forwards the
packets back to the originating end. This mode monitors link status in both directions.
BFD operating modes
Before a BFD session is established, BFD has two operating modes, active and passive.
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Active mode: In this mode, BFD actively sends BFD control packets regardless of whether any BFD
control packet is received from the peer.
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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 BFD operating modes:
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Asynchronous mode—both endpoints periodically send BFD control packets to each other. BFD
considers that the session is down if it receives no BFD control packets within a specific interval.
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Demand mode—no BFD control packets are exchanged after the session is established. It is
assumed that the endpoints have another way to verify connectivity to each other. However, either
host may still send BFD control packets if needed.
NOTE:
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Only asynchronous mode is supported.
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When a BFD session is maintained by sending Echo packets, the session is independent of the operating
mode.
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When 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 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.
BFD packet format
BFD control packets are encapsulated into UDP packets with port number 3784 for single-hop detection
or port number 4784 for multi-hop detection (also can be 3784 based on the configuration task). BFD
