Principles and applications of BFD technology

I. Introduction to BFD

BFD (Bidirectional Forwarding Detection) is a unified detection mechanism for the entire network. It is used to quickly detect and monitor the forwarding connectivity of links or IP routes in the network.

2. In order to improve the performance of the existing network, the neighbors must be able to quickly detect the communication failure, thus establishing an alternate channel to resume communication.

Second, commonly used fault detection methods

1. Hardware detection: For example, an SDH (Synchronous Digital Hierarchy) alarm detects link faults. The advantage of hardware detection is that faults can be quickly detected, but not all media provide hardware detection.

2. Slow Hello mechanism: The Hello message mechanism in the routing protocol is usually adopted. The time required for this mechanism to detect a fault is in the order of seconds. Detection time of more than 1 second for high-speed data transmission (such as gigabit rate) will result in a large amount of data loss; delays of more than 1 second for delay-sensitive services (such as voice services) are also unacceptable. Also, this mechanism relies on routing protocols.

3. Other detection mechanisms: Different protocols sometimes provide dedicated detection mechanisms, but such dedicated detection mechanisms are often difficult to deploy when inter-system connectivity.

Third, the working mechanism of BFD

1 Overview

1 BFD provides a universal, standardized, medium-independent, protocol-independent fast fault detection mechanism that can quickly detect the failure of bidirectional forwarding paths between two routers for each upper layer protocol such as routing protocol and MPLS.  There are some ccna dumps pdf for you to improve yourselves.

2 BFD establishes a session on two routers to monitor the bidirectional forwarding path between the two routers and serve the upper layer protocol.

3 BFD does not discover the mechanism itself. Instead, it is notified by the upper layer protocol of the service that it has established a session. If the BFD control packet is not received within the detection time after the session is established, the BFD control packet is considered to be faulty. Protocol, upper layer protocol for the corresponding processing

3. The detection method of BFD

1 Single-hop detection: BFD single-hop detection refers to IP connectivity detection for two directly connected systems. The “single-hop” mentioned here is a hop of IP.

2 Multi-hop detection: BFD can detect any path between two systems, which may span many hops or overlap in some parts.

3 Bidirectional detection: BFD detects the link status in both directions by transmitting detection packets at both ends of the bidirectional link, and implements link fault detection in milliseconds. BFD detection of LSPs is a special case. The BFD control packet is sent in the direction, and the peer reports the link status through other paths.

4. How the BFD session works

1 Control packet mode: Sessions at both ends of the link monitor link status through control packets.

2 Echo packet mode: One end of the link forwards the Echo packet and is forwarded by the other end to implement bidirectional monitoring of the link.

5, BFD operating mode

1 Active mode: Proactively sends BFD control packets before receiving a BFD control packet from the peer before the session is established.

2 Passive mode: The BFD control packet is not sent before the session is established until the control packet is sent from the peer.


1> During session initialization, at least one of the communicating parties must run in active mode to successfully establish a session.

2> There are two modes after the BFD session is established: asynchronous mode and query mode.

3> Both sides of the communication are required to run in the same mode [currently only supports asynchronous mode]

An Asynchronous mode: The device running in the asynchronous mode periodically sends BFD control packets. If the peer does not receive BFD control packets within the detection time, the session is considered down.

B Query mode: Assume that there is a separate method to confirm the connectivity between itself and the peer system. After the BFD session is established, the BFD control packet will be sent periodically, unless the connectivity needs to be verified explicitly.

6. BFD authentication method

1 Simple: Simple character authentication

2 MD5: MD5 certification

3 SHA1: SHA1 Authentication (Secure Hash Algorithm 1)

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