Technical Tip: Behavior of the BFD and underlying routing protocol when BFD is disabled

[smaruvala]

Description	This article explains the behaviour of BFD and the underlying routing protocol when the Administrator disables BFD in a neighbor. It also explains whether disabling it at one end of the network has an impact.
Scope	FortiGate.
Solution	Bidirectional Forwarding Detection (BFD) is a protocol used with Routing protocols such as BGP and OSPF to detect path failures and is used for fast convergence of routing protocols. If BFD is enabled in 2 neighbor devices in BGP neighborship, disabling the BFD configuration in one end does not bring the Routing protocol down. In the below example, BFD is configured along with the BGP, and the status of the BFD is shown as 'UP'. kvm126 # get router info bfd neighbor OurAddress    NeighAddress  State   Interface  LDesc/RDesc 10.173.18.190 10.173.5.4     UP       port2     18/3 When an Administrator disables the BFD on one end, then that specific device will send a BFD packet with 'Diagnostic Code' as 0x07 (Administratively Down) and 'Session State' as 0x0 (AdminDown). When FortiGate receives this packet, FortiGate will only change the BFD state from UP to DOWN. It will not bring the BGP down. kvm126 # get router info bfd neighbor OurAddress     NeighAddress   State  Interface LDesc/RDesc 10.173.18.190   10.173.5.4     DOWN    port2     18/3 kvm126 # get router info bgp neighbors VRF 0 neighbor table: BGP neighbor is 10.173.5.4, remote AS 2000, local AS 1000, external link BGP version 4, remote router ID 10.182.5.4 BGP state = Established, up for 00:53:19 ---> BGP up time is not changed to zero Related article: Technical Tip: FortiGate BFD implementation and examples (Bidirectional Forwarding Detection for OSP...