Do you know what BFD is? This article explains the principle and usage scenarios of BFD protocol in detail

With the wide deployment of network applications , Network failure may lead to abnormal business . To reduce the link 、 The impact of equipment failure on business , Improve the reliability of the network , The network equipment needs to detect the communication failure with adjacent equipment as soon as possible , So as to take measures in time , To ensure the normal operation of the business .

BFD（Bidirectional Forwarding Detection, Two way forwarding detection ） Provides a generic 、 standardized 、 Media independent and protocol independent fast fault detection mechanism , For rapid detection 、 Monitor the links in the network or IP Forwarding connectivity status of the route .

In a system where a failure cannot be detected by a hardware signal , Applications usually use the upper layer protocol itself Hello Message mechanism detects network failure .

Common routing protocols Hello The message mechanism takes a long time to detect , The detection time exceeds 1 Second . When the data transmitted by the application in the network exceeds GB/s when , The detection time of seconds will lead to a large amount of loss of data transmitted by the application .

In a three-tier network , Static routing itself has no fault checking mechanism .

BFD Provides a generic 、 standardized 、 Media independent 、 Protocol independent fast fault detection mechanism , It has the following two advantages ：

Provide light load for channels between adjacent forwarding engines 、 Fast fault detection .

Use a single mechanism for any medium 、 Any protocol layer performs real-time detection .

BFD It's a simple one “Hello” agreement . Establish... Between the two systems BFD Session channel , And send it periodically BFD Detection message , If a system does not receive the detection message from the opposite end within the specified time , It is considered that some part of the channel has failed .

BFD Detection is established between the two systems through maintenance BFD Session , The system sends BFD Message establishment session .

BFD The control message is encapsulated differently according to different scenarios , The message structure consists of mandatory part and optional authentication field .

BFD There are two ways to establish a session , Static establishment, i.e BFD Session and dynamic establishment BFD conversation .BFD Different sessions are distinguished by the local identifier and the remote identifier in the control message . Static and dynamic creation BFD The main difference in conversation is Local Discriminator and Remote Discriminator The configuration methods are different .

BFD Use scenarios

Static routing itself has no detection mechanism , If the static route has redundant paths , Through static routing and BFD linkage , When the primary path fails , Realize fast switching of static routes .

Static routing vs BFD Linkage is widely used , As shown in the figure below R1 It is the exit router of the campus network ,R1 Connect through two links ISP1 and ISP2, Under normal circumstances, the default route passes through a link that points to ISP1 The link to , When it comes to ISP1 When the link fails ,BFD The conversation can quickly perceive , And notify the router to switch the traffic to ISP2 The link to .

OSPF In unbound BFD Under the circumstances , The link fault detection time is determined by the protocol Hello The mechanism decides , Usually in seconds . By binding BFD, Millisecond level fault detection can be realized .

BFD And OSPF Linkage means that BFD and OSPF The agreement is linked , BFD Inform... Of the quick detection result of link failure OSPF agreement .