H3C Technologies H3C S7500E Series Switches - Manual (Page 175)

11-3
3) BFD notifies the protocol of the failure.
4) The protocol terminates the neighborship on the link.
5) If a backup link is available, the protocol will use it to forward packets.
No detection time resolution is defined in the BFD draft. At present, most devices supporting
BFD provide detection measured in milliseconds.
BFD detection methods
z Single-hop detection: Detects the IP connectivity between two directly connected systems.
Single hop means one hop for IP forwarding.
z Multi-hop detection: Detects any of the paths between two systems. These paths may have
multiple hops and maybe overlapped.
z 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
z Control packet mode: Both ends of the link exchange BFD control packets to monitor link
status.
z 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.
z Active mode: In this mode, BFD actively sends BFD control packets regardless of whether
any BFD control packet is received from the peer.
z 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 demand.
z Asynchronous mode: In 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.
z Demand mode: In 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.