Reference Guide

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Bidirectional Forwarding Detection (BFD)
Bidirectional forwarding detection (BFD) is supported only on the S4820T platform.
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 module (RPM). Only session state changes are reported to the BFD
Manager (on the RPM), 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 functionality will be implemented in phases. On the S4820T platform, BFD is supported on 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 (FTOS) 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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