Bidirectional Forwarding Detection

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Bidirectional Forwarding Detection , or BFD for short , is a minimalistic connection-oriented network protocol for the detection of errors between two routers connected to one another .

It was developed to shorten the convergence time in networks in the event of faults. The notification of status changes in IP networks is possible with BFD in the sub-second range.

BFD works with a low protocol overhead even on media that have no or only a limited or sluggish error detection option. These include, for example, Virtual Circuits (VC) in the SDH environment, IP tunnel connections and MPLS label-switched paths (LSP).

BFD establishes a connection ( session ) between two end points (routers). If these are not directly adjacent, one speaks of multihop BFD.

The BFD connection is preceded by a three-way handshake .

The authentication of BFD sessions is possible. Depending on the device manufacturer and firmware version, not only passwords in plain text but also MD5 or SHA1- based methods are available.

The end-to-end availability is checked by periodically sent Hello messages. Hello intervals are manufacturer and device-specific and are in the range of a few milliseconds. If the third Hello message is not received, the BFD session is aborted by default.

With the option of revealing the status of connections to routing protocols such as OSPF and IS-IS, they are able to detect the failure of their neighbors even below the dead-timer time typical for the IGP . This even applies to the Routing Information Protocol (RIP), which by default uses a 30-second value for its hello time interval. For EGPs , the BGP protocol can use the status information from BFD in order to achieve fast convergence times. In the case of static routes, the BFD provides a tool with which the accessibility of the “next hop” of the route can be monitored and , if it cannot be reached, it can be removed from the routing table .

In contrast to routing protocols such as RIP , OSPF or IS-IS, BFD has no mechanism for the automatic detection of neighboring routers. Therefore, each BFD session between routers must be configured accordingly.

Market-leading network component manufacturers enable the processing of BFD sessions in hardware on the line card of their router. The number of supported BFD sessions depends on the device and ranges from less than a hundred to over a thousand BFD sessions. Depending on the router model, authentication can be used to process the BFD sessions in software by the router CPU , thereby limiting the number of possible BFD sessions or increasing the BFD Hello intervals.

specification

BFD is described in RFC 5880 for the detection of MPLS-LSP errors.

The interaction between BFD and the routing protocols OSPF and IS-IS was described in RFC 5881 .

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