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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.

I wanted to write about BFD a year ago when it was first advertised as being available in the low-end routers (BFD support on high-end platforms is much better, but I simply don’t have a GSR and a CRS-1 at home … yet), but it failed to work, so I had to shelve the idea until the IOS release 12.4(15)T matured to a point where BFD on ISR started working in IOS, not just in Powerpoint.

In this month’s IP corner article, “Improve the Convergence of Mission-Critical Networks with Bidirectional Forwarding Detection (BFD)”, I’m describing BFD principles, its configuration on Cisco IOS and give you practical examples how you can use BFD to improve next-hop failure detection.


  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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