Bidirectional Forwarding Detection (Bfd); How Bfd Works - Dell S6000–ON Configuration Manual

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Bidirectional Forwarding Detection (BFD)

BFD is a protocol that is used to rapidly detect communication failures between two adjacent systems. It
is a simple and lightweight replacement for existing routing protocol link state detection mechanisms. It
also provides a failure detection solution for links on which no routing protocol is used.
BFD is a simple hello mechanism. Two neighboring systems running BFD establish a session using a
three-way handshake. After the session has been established, the systems exchange periodic control
packets at sub-second intervals. If a system does not receive a hello packet within a specified amount of
time, routing protocols are notified that the forwarding path is down.
BFD provides forwarding path failure detection times on the order of milliseconds rather than seconds as
with conventional routing protocol hellos. It is independent of routing protocols, and as such, provides a
consistent method of failure detection when used across a network. Networks converge faster because
BFD triggers link state changes in the routing protocol sooner and more consistently because BFD
eliminates the use of multiple protocol-dependent timers and methods.
BFD also carries less overhead than routing protocol hello mechanisms. Control packets can be
encapsulated in any form that is convenient, and, on Dell Networking routers, BFD agents maintain
sessions that reside on the line card, which frees resources on the route processor. Only session state
changes are reported to the BFD Manager (on the route processor), which in turn notifies the routing
protocols that are registered with it.
BFD is an independent and generic protocol, which all media, topologies, and routing protocols can
support using any encapsulation. Dell Networking has implemented BFD at Layer 3 and with user
datagram protocol (UDP) encapsulation. BFD is supported on static routing protocols and dynamic
routing protocols such as VRRP, OSPF, OSPFv3, IS-IS, and BGP.

How BFD Works

Two neighboring systems running BFD establish a session using a three-way handshake.
After the session has been established, the systems exchange control packets at agreed upon intervals. In
addition, systems send a control packet anytime there is a state change or change in a session parameter.
These control packets are sent without regard to transmit and receive intervals.
NOTE: The Dell Networking Operating System (OS) does not support multi-hop BFD sessions.
If a system does not receive a control packet within an agreed-upon amount of time, the BFD agent
changes the session state to Down. It then notifies the BFD manager of the change and sends a control
packet to the neighbor that indicates the state change (though it might not be received if the link or
receiving interface is faulty). The BFD manager notifies the routing protocols that are registered with it
(clients) that the forwarding path is down and a link state change is triggered in all protocols.
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Bidirectional Forwarding Detection (BFD)