H3C S9500E Series Configuration Manual page 172

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 and find link failures in milliseconds. (BFD LSP detection is a
special case, in which BFD control packets are sent in one direction, and the peer switch
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 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 switch operating in 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 to verify connectivity explicitly in
response to network conditions.
Only asynchronous mode is supported.
•
When a BFD session operates in Echo mode, the session is independent of the operating mode.
•
172