R3303-HP HSR6800 Routers High Availability Configuration Guide

203
• 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 might 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
Session modes for BFD include:
• Control packet mode—Both ends of the link exchange BFD control packets to monitor link status.
• Echo packet 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 two 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 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:
• 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.
• 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 might still send BFD control packets if needed.
When the 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.
NOTE:
• Only the asynchronous mode is supported.
• When a BFD session is maintained by sending Echo packets, the session is independent of the operatin
g
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 (also can be 3784 based on the configuration task). BFD