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FTOS Configuration Guide for the C-Series - Force10 Networks

FTOS Configuration Guide for the C-Series - Force10 Networks

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How BFD Works<br />

Two neighboring systems running BFD establish a session using a three-way handshake. After <strong>the</strong> session<br />

has been established, <strong>the</strong> systems exchange control packets at agreed upon intervals. In addition, systems<br />

send a control packet anytime <strong>the</strong>re is a state change or change in a session parameter; <strong>the</strong>se control<br />

packets are sent without regard to transmit and receive intervals.<br />

Note: <strong>FTOS</strong> does not support multi-hop BFD sessions.<br />

If a system does not receive a control packet within an agreed-upon amount of time, <strong>the</strong> BFD Agent<br />

changes <strong>the</strong> session state to Down. It <strong>the</strong>n notifies <strong>the</strong> BFD Manager of <strong>the</strong> change, and sends a control<br />

packet to <strong>the</strong> neighbor that indicates <strong>the</strong> state change (though it might not be received if <strong>the</strong> link or<br />

receiving interface is faulty). The BFD Manager notifies <strong>the</strong> routing protocols that are registered with it<br />

(clients) that <strong>the</strong> <strong>for</strong>warding path is down, and a link state change is triggered in all protocols.<br />

Note: A session state change from Up to Down is <strong>the</strong> only state change that triggers a link state change in<br />

<strong>the</strong> routing protocol client.<br />

BFD packet <strong>for</strong>mat<br />

Control packets are encapsulated in UDP packets. Figure 419 shows <strong>the</strong> complete encapsulation of a BFD<br />

control packet inside an IPv4 packet.<br />

598 Bidirectional Forwarding Detection

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