Bidirectional Forwarding Detection (BFD) Protocol Cheat Sheet
Definition This is a protocol initially described inRFC5880and IPv4/IPv6 specifics inRFC5881. I would say this is an aggressive 'hello-like' protocol with shorter timers but very lightweight on the wire and requiring very little processing as it is designed to be implemented in forwarding plane (although RFC does not forbid it to be implemented in control plane). It also contains a feature called Echo that further leaves cpu processing cycle to roughly ZERO which literally just 'loops' BFD control packets sent from peer back to them without even 'touching' (processing) it. BFD helps routing protocol detects peers failure at sub-second level and BIG-IP supports it on all its routing protocols. On BIG-IP it is control-plane independent as TMM that takes care of BFD sending/receivingunicastprobes (yes,no multicast!) and BIG-IP's Advanced Routing Module® being responsible only for its configuration (of course!) and to receive state information from TMM that is displayed in show commands. BIG-IP's control plane daemon communicates with TMM isoamd. This daemon starts when BFD is enabled in the route domain like any other routing protocol. BFD Handshake Explained 218: BFD was configured on Cisco Router but not on BIG-IP so neighbour signals BIG-IP sessionstate is Down and no flags 219: I had just enabled BFD on BIG-IP, session state is now Init and only Control Plane Independent flag set¹ 220: Poll flag is set to validate initial bidirectional connectivity (expecting Final flag set in response) 221: BIG-IP sets Final flag and handshake is complete² ¹Control Plane Independentflag is set because BFD is not actively performed by BIG-IP's control plane. BIG-IP's BFD control plane daemon (oamd) just signals TMM what BFD sessions are required and TMM takes care of sending/receiving of all BFD control traffic and informs session state back to Advanced Routing Module's daemon. ²Packets 222-223are just to show that after handshake is finished all flags are cleared unless there is another event that triggers them. Packet 218 is how Cisco Router sees BIG-IP when BFD is not enabled. Control Plane Independent flag on BIG-IP remains though for the reasons already explained above. Protocol fields Diagnostic codes 0 (No Diagnostic): Typically seen when BFD session is UP and there are no errors. 1 (Control Detection Time Expired):BFD Detect Timer expired and session was marked down. 2 (Echo Function Failed):BFD Echo packet loop verification failed, session is marked down and this is the diagnostic code number. 3 (Neighbor Signaled Session Down):If either neighbour advertised state or our local state is down you will see this diagnostic code 4 (Forwarding Planet Reset):When forwarding-plane (TMM) is reset and peer should not rely on BFD so session is marked down. 5 (Path Down):Ondemand mode external application can signal BFD that path is down so we can set session down using this code 6 Concatenated Path Down): 7 (Administratively Down):Session is marked down by an administrator 8 (Reverse Concatenated Path Down): 9-31:Reserved for future use BFD verification 'show' commands ³Type IP address to see specific session Modes Asynchronous(default): hello-like mode where BIG-IP periodically sends (and receives) BFD control packets and if control detection timer expires, session is marked as down.It uses UDP port3784. Demand: BFD handshake is completed but no periodic BFD control packets are exchanged as this mode assumes system has its own way of verifying connectivity with peer and may trigger BFD verification on demand, i.e. when it needs to use it according to its implementation.BIG-IP currently does not support this mode. Asynchronous + Echo Function: When enabled, TMM literally loops BFDecho-specificcontrol packetson UDP port 3785sent from peers back to them without processing it as it wasn't enough that this protocol is already lightweight. In this mode, a less aggressive timer (> 1 second) should be used for regularBFD control packets over port 3784 and more aggressive timer is used by echo function.BIG-IP currently does not support this mode. Header Fields Protocol Version:BFD version used. Latest one is v1 (RFC5880) Diagnostic Code: BFD error code for diagnostics purpose. Session State: How transmitting system sees the session state which can be AdminDown, Down, Init or Up. Message Flags:Additional session configuration or functionality (e.g. flag that says authentication is enabled) Detect Time Multiplier:Informs remote peer BFD session is supposed to be marked down ifDesired Min TX Intervalmultiplied by this value is reached Message Length(bytes):Length of BFD Control packet My Discriminator:For each BFD session each peer will use a unique discriminator to differentiate multiple session. Your Discriminator:When BIG-IP receives BFD control message back from its peer we add peer's My Discriminator to Your Discriminator in our header. Desired Min TX Interval(microseconds):Fastest we can send BFD control packets to remote peer (no less than configured value here) Required Min RX Interval(microseconds):Fastest we can receive BFD control packets from remote peer (no less than configured value here) Required Min Echo Interval(microseconds):Fastest we can loop BFD echo packets back to remote system (0 means Echo function is disabled) Session State AdminDown:Administratively forced down by command Down: Either control detection time expired in an already established BFD session or it never came up.If probing time (min_tx) is set to 100ms for example, and multiplier is 3 then no response after 300ms makes system go down. Init:Signals a desire to bring session up in the beginning of BFD handshake. Up:Indicates session is Up Message Flags Poll:Pool flag is just a 'ping' that requires peer box to respond with Final flag. In BFD handshake as well as in Demand mode pool message is a request to validate bidirectional connectivity. Final:Sent in response to packet with Poll bit set Control Plane Independent:Set if BFD can continue to function if control plane is disrupted¹ Authentication Present:Only set if authentication is being used Demand:If set, it is implied that periodic BFD control packets are no longer sent and another mechanism (on demand) is used instead. Multipoint:Reserved for future use of point-to-multipoint extension. Should be 0 on both sides. ¹ This is the case for BIG-IP as BFD is implemented in forwarding plane (TMM) BFD Configuration Configure desired transmit and receive intervals as well as multiplier on BIG-IP. And Cisco Router: You will typically configure the above regardless of routing protocol used. BFD BGP Configuration And Cisco Router: BFD OSPFv2/v3 Configuration BFD ISIS Configuration BFD RIPv1/v2 Configuration BFD Static Configuration All interfaces no matter what: Specific interface only: Tie BFD configuration to static route:
