| Description | This article describes 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 example below, BFD is configured along with BGP, and the status of the BFD is shown as 'UP'.
kvm126 # get router info bfd neighbor OurAddress NeighAddress State Interface LDesc/RDesc
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 kvm126 # get router info bgp neighbors
Note that the diagnostic code in the log, which indicates the BFD session is 'Administratively Down', is 0x27:
state UP -> ADMIN DOWN local_diag=0x27
0x20 is a locally defined value and won't be transmitted to the peer, which also means that FortiGate will not send a notification to routing daemons.
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