Bidirectional Forwarding Detection (Bfd); How Bfd Works - Dell S4048–ON Configuration Manual

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.
NOTE: A session state change from Up to Down is the only state change that triggers a link state change in the routing
protocol client.
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