H3C S3610-28P Operation Manual page 560

Operation Manual – BFD-GR
H3C S3610&S5510 Series Ethernet Switches
routers to which it needs to establish sessions. After a session is established, if no BFD
control packet is received from the peer within the negotiated BFD interval, BFD notifies
a failure to the upper-layer, which takes appropriate measures.
I. How BFD interacts with an application-layer protocol
After discovering neighbors by using its own neighbor discovery mechanism, an
application-layer protocol notifies BFD of the protocol type, neighbor address,
interface, and whether the neighbor is directly connected. Upon receiving the
notification, BFD sends control packets to establish sessions.
When BFD is disabled or a neighbor is removed, BFD receives a notification from
the application-layer protocol to tear down the session. If no other applications
need to monitor this link, BFD tears down the corresponding session.
II. Failure detection mechanism
After a BFD session is established, if no BFD control packet is received from the
neighbor within the BFD interval, BFD sets the session state to down and notifies it to
the protocol concerned.
Upon receiving the link failure notification from BFD, the application-layer protocol
considers the neighbor down.
Note:
No detection time resolution is defined in the BFD draft. At present, most devices
supporting BFD provide detection measured in milliseconds.
III. 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.
IV. BFD operation modes
Before a BFD session is established, there are two BFD operation modes: active or
passive.
Active mode: Before a session is established, BFD actively sends BFD control
packets regardless of whether any BFD control packet is received from the peer.
Passive mode: Before a session is established, no BFD control packet is sent until
a BFD control packet is received from the peer.
1-2
Chapter 1 BFD Configuration