Technical Tip: How to clear BFD Sessions without disturbing the BGP sessions

[srajeswaran]

Description	This article describes how to remove BFD sessions without impacting the BGP sessions.
Scope	FortiGate.
Solution	Consider the following topology: BGP and BFD is up between FortiGate-1 and FortiGate-2. FortiGate-2 # get router info bgp summary VRF 0 BGP router identifier 10.10.10.2, local AS number 12345 BGP table version is 1 3 BGP AS-PATH entries 0 BGP community entries Neighbor    V  AS     MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.10.10.1  4  12346  14      14      1      0   0    00:10:01      1 Total number of neighbors 1 FortiGate-2 # get router info bfd neighbor OurAddress NeighAddress State Interface LDesc/RDesc 10.10.10.2 10.10.10.1   UP    port4         4/2 To remove the BFD dependency on BGP, configure 'set bfd disable' under the BGP neighbor configuration. This will bring down the BFD neighborship, but will not bring down BGP as BFD uses 'Diagnostic Code: Administratively Down (0x07)'. This informs the neighbor that BFD is administratively disabled and not down due to reasons like keepalive being missing. FortiGate-2 # get router info bfd neighbor OurAddress NeighAddress State Interface LDesc/RDesc 10.10.10.2 10.10.10.1   DOWN  port4         7/6 FortiGate-2 # get router info bgp summary VRF 0 BGP router identifier 10.10.10.2, local AS number 12345 BGP table version is 2 3 BGP AS-PATH entries 0 BGP community entries Neighbor    V   AS    MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.10.10.1  4   12346 228     233     1      0   0 0  1:02:44    2    <----- BGP stays UP even though BFD is down. Total number of neighbors 1 Admin Down PCAP: When keepalives are missed, BFD sends 'Diagnostic Code: Control Detection Time Expired (0x01)'. This will trigger BGP to go down. Keepalive missing PCAP: Related article: Technical Tip: FortiGate BFD implementation and examples (Bidirectional Forwarding Detection for OSP...