Bidirectional Forwarding Detection(BFD)BFD is a protocol that is used to rapidly detect communication failures between two adjacent systems. It is asimple and lightweight replacement for existing routing protocol link state detection mechanisms. It alsoprovides 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 atsub-second intervals. If a system does not receive a hello packet within a specified amount of time, routingprotocols are notified that the forwarding path is down.BFD provides forwarding path failure detection times on the order of milliseconds rather than seconds as withconventional routing protocol hellos. It is independent of routing protocols, and as such, provides aconsistent method of failure detection when used across a network. Networks converge faster because BFDtriggers link state changes in the routing protocol sooner and more consistently because BFD eliminates theuse of multiple protocol-dependent timers and methods.BFD also carries less overhead than routing protocol hello mechanisms. Control packets can be encapsulatedin any form that is convenient, and, on Dell Networking routers, BFD agents maintain sessions that reside onthe line card, which frees resources on the Route Processor. Only session state changes are reported to theBFD 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 supportusing any encapsulation. Dell Networking has implemented BFD at Layer 3 and with user datagram protocol(UDP) encapsulation. BFD functionality will be implemented in phases. On the switch, BFD is supported onstatic routes and dynamic routing protocols, such as VRRP, OSPF, OSPFv3, IS-IS, and BGP.Topics:• How BFD Works• Important Points to Remember• Configure BFDHow BFD WorksTwo 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. Inaddition, 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.7Bidirectional Forwarding Detection (BFD) 164
