Bidirectional Forwarding Detection (BFD) protocol has finally been published as a series of RFCs. BFD gives you quick failure detection between L3 hops (routers) regardless of the underlying technology and equipment (modems, media converters, bridges). It’s been gradually introduced in Cisco IOS during the last few years; release 15.0M and 12.2SRE contain almost everything you’ll ever need (missing: multihop BGP support and MPLS LSP support).

I wrote about BFD in Improve the Convergence of Mission-Critical Networks with Bidirectional Forwarding Detection (BFD) article (you’ll find it somewhere in this list). To learn more, read the RFCs in this order:

• When, where and how should BFD be used (RFC 5882). This one is mandatory; it gives you a good overview of what BFD can and cannot do as well as where and how to use it.
• BFD: the protocol (RFC 5880). The protocol description. Not as boring as some other protocol descriptions are. If you want to understand how the Echo function works, make sure you read this one.
• BFD for single-hop IPv4 and IPv6 (RFC 5881). The dirty details of using BFD for single-hop IPv4 and IPv6 failure detection.
• Multihop BFD (RFC 5883). Extensions to RFC 5881 needed to support BFD in multihop scenarios (IBGP sessions, multihop EBGP sessions, OSPF virtual links).
• BFD for MPLS LSP (RFC 5884). This one allows you to test PE-to-PE forwarding path. Ideal for very quick rerouting around data-plane failures in MPLS VPN networks (not implemented in IOS yet).
• BFD for pseudowires (RFC 5885). Allows you to test data-plane consistency of pseudowires established across your MPLS network.