Bidirectional Forwarding Detection

BFD is one of those simple ingenious ideas that make you wonder “Why did it take them so long to figure this out?” It’s a UDP-based protocol that replaces dozens of link-level failure-detection mechanisms and routing protocol tweaks with a simple, focused solution: detect hop-by-hop layer-3 failures.

In November 2008 IP corner article I described BFD principles, its configuration on Cisco IOS and give you practical examples how you can use BFD to improve next-hop failure detection. You’ll find the link to the article somewhere in this list.

For more details on how BFD interacts with the routing protocols watch the How Networks Really Work webinar.


  1. BFD is a good enabler for OAM starved protocols (Ethernet), but buys you slightly less for a SONET/SDH transport based IP network. Consider an ultra long haul link and how quickly a SONET component can signal interface down and hence propagate that to the IGP. I say only slightly less, because BFD helps a tad against soft failures. Most people don't monitor their interface counters as religiously as they should. BFD helps safeguard slightly against this. Now, if you take a large scale network where link aggregation is necessary and BFD breaks down instantly because it hashes to one link. Moral is, BFD is helpful but no pancea.
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