H3C Technologies H3C S10500 Series Switches - Manual (Page 171)

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BFD detection methods
• Single-hop detection—Detects the IP connectivity between two directly connected systems.
• Multi-hop detection—Detects any of the paths between two systems. These paths have multiple hops
and may be overlapped.
• 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
• 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, monitoring link status in both directions.
BFD operating modes
Before a BFD session is established, BFD has the following operating modes—active and passive.
• Active mode—BFD actively sends BFD control packets regardless of whether any BFD control packet
is received from the peer.
• Passive 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 the asynchronous mode—Both endpoints
periodically send BFD control packets to each other. BFD considers the session down if it receives no BFD
control packets within a specific interval.
NOTE:
When a BFD session is maintained by sending Echo packets, the session is independent of the operating
mode.
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 (it can also be 3784 based on the configuration task). BFD
echo packets have a similar format to BFD control packets (except that the Desired Min TX Interval and
Required Min RX Interval fields are null), with UDP port number 3785.