H3C Technologies H3C S6800 Series Switches - Manual (Page 56)

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• Control packets—Encapsulated into UDP packets with port number 3784 for single-hop detection
or port number 4784 for multi-hop detection.
Echo packet mode
The local end of the link sends echo packets to establish BFD sessions and monitor link status. The peer
end does not establish BFD sessions and only forwards the packets back to the originating end.
In echo packet mode, BFD supports only single-hop detection and the BFD session is independent of the
operating mode.
Control packet mode
Both ends of the link exchange BFD control packets to monitor link status.
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. 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 the connectivity is found to be up, no
more BFD control packets are sent until the next command is issued.
In addition, both ends of the link can exchange BFD control packets to establish and maintain BFD
sessions, and one end of the link sends echo packets to monitor link status.
Supported features
• Static routing. For more information, see Layer 3—IP Routing Configuration Guide.
• IPv6 static routing. For more information, see Layer 3—IP Routing Configuration Guide.
• OSPF. For more information, see Layer 3—IP Routing Configuration Guide.
• OSPFv3. For more information, see Layer 3—IP Routing Configuration Guide.
• BGP. For more information, see Layer 3—IP Routing Configuration Guide.
• IPv6 BGP. For more information, see Layer 3—IP Routing Configuration Guide.
Protocols and standards
• RFC 5880, Bidirectional Forwarding Detection (BFD)
• RFC 5881, Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)
• RFC 5882, Generic Application of Bidirectional Forwarding Detection (BFD)
• RFC 5883, Bidirectional Forwarding Detection (BFD) for Multihop Paths