Users Guide

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 the sessions. Only

session state changes are reported to the BFD Manager, 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 S5000 platform, BFD is

supported on dynamic routing protocols such as OSPF, IS-IS, and BGP.

Topics:

• How BFD Works

• BFD Packet Format

• BFD Sessions

• BFD Three-Way Handshake

• Session State Changes

• Configure BFD

• Troubleshooting BFD

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