Bidirectional Forwarding Detection (BFD)

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By Ivan Pepelnjak

Bidirectional Forwarding Detection (BFD) is a UDP-based layer-3 protocol that provides very fast routing protocol independent detection of layer-3 next hop failures.

BFD can be used to replace the routing protocol timer tweaks with a fast and reliable failure detection mechanism. In networks that use multiple routing protocols over the same layer-3 interface, a single BFD instance can serve numerous routing protocols and provide peer failure detection to all of them.

Once the routing protocols are configured to support BFD, the interaction between BFD and routing protocols is automatic. When the routing protocols detect a new neighbor, they pass the information to BFD which tries to establish a BFD adjacency with the request IP address and reports the peer status back to the routing protocols if the BFD adjacency has been successfully established.

High-end platforms implement BFD on the line cards (not on the main CPU or supervisory board) resulting in highly scalable failure detection mechanism.

Distributed BFD operation

The BFD Echo mode can be used to further optimize the line card CPU utilization. When the Echo mode is successfully negotiated, BFD extends the interval between control packets that have to be processed by BFD neighbors. The regular keepalive packets are replaced with IP packets address to the router’s own IP address but sent to the neighbor’s layer-2 address (thus burdening only the neighbor’s L3 forwarding logic).

BFD echo mode

Read more in the Improve the Convergence of Mission-Critical Networks with BFD IP corner article ...

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