Reference Guide

148 | Bidirectional Forwarding Detection (BFD)
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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, sessions are maintained by
BFD Agents. 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 UDP
encapsulation. BFD functionality will be implemented in phases. On the S5000, BFD is supported on
dynamic routing protocols such as OSPF, 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.
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.
BFD packet format
Control packets are encapsulated in UDP packets. The following illustration shows the complete
encapsulation of a BFD control packet inside an IPv4 packet.
Note: FTOS does not support multi-hop BFD sessions.
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.