H3C Technologies H3C MSR 5600 - Manual (Page 57)

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Configuring BFD
Introduction to BFD
Bidirectional forwarding detection (BFD) provides a general-purpose, standard, medium- and
protocol-independent fast failure detection mechanism. It can detect and monitor the connectivity of links
in IP to detect communication failures quickly so that measures can be taken to ensure service continuity
and enhance network availability.
BFD can uniformly and quickly detect the failures of the bidirectional forwarding paths between two
devices for upper-layer protocols such as routing protocols and MPLS. The hello mechanism used by
upper layer protocols needs seconds to detect a link failure, while BFD can provide detection measured
in milliseconds.
BFD can be used for single-hop and multi-hop detections:
• 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.
BFD session establishment
BFD provides no neighbor discovery mechanism. Protocols that BFD services notify BFD of routers to
which it needs to establish sessions.
BFD sessions are established as follows:
1. A protocol sends Hello messages to discover neighbors and establish neighborships.
2. After establishing neighborships, the protocol notifies BFD of the neighbor information, including
destination and source addresses.
3. BFD uses the information to establish BFD sessions.
When BFD detects a link failure:
4. BFD clears the neighbor session.
5. BFD notifies the protocol of the failure.
6. The protocol terminates the neighborship on the link.
7. If a backup link is available, the protocol will use it for communication.
BFD session modes and operating modes
BFD sessions use the following types of packets:
• Echo packets—Encapsulated into UDP packets with port number 3785.
• Control packets—Encapsulated into UDP packets with port number 3784 for single-hop detection
or port number 4784 for multi-hop detection.