Implement and troubleshoot bidirectional forwarding detection (BFD)

BFD – RFC 7419

BFD is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols.

BFD is not tied to any routing protocol. A routing protocol can utilize BFD to held detect neighbor failures faster. Enabled at an interface level. Must be configured on both ends of the link

CEF and IP routing is required on the router

Used to detect faults between 2 nodes connected by a link

• Low overhead detection on physical media that doesn’t support failure detection
• 3 way handshake to establish session
• Supports authentication
• Must be explicitly configured

Modes

• Asynchronous
  • Periodically send Hellos between each other
  • If number of packets are not received, session is considered down
• Demand
  • No hellos are exchanged after session is established
  • Assumed endpoints have another way to verify connectivity

Echo mode is enabled by default, works with asynchronous BFD

Config

interface [interface]
bfd internal [ms] mix-rx [ms] multiplier [interval]
bfd interval 50 min_rx 50 multiplier 5 

router bgp [as]
neighbor [ip] fall-over bfd

router eigrp [as]
bfd all-interfaces

router ospf [pid]
bfd all-interfaces

https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html

https://en.wikipedia.org/wiki/Bidirectional_Forwarding_Detection