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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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Bidirectional Forwarding Detection (BFD) is a mechanism to detect communication faults between forwarding engines.

To be specific, BFD detects connectivity of a data protocol on a path between two systems. The path can be a physical link, a logical link, or a tunnel.

In BFD for OSPF, a BFD session is associated with OSPF. The BFD session quickly detects a link fault and then notifies OSPF of the fault. This speeds up OSPF's response to the change of the network topology.


The link fault or the topology change may cause devices to re-calculate routes. Therefore, the convergence of routing protocols must be as quick as possible to improve the network performance.

Link faults are unavoidable. Therefore, a feasible solution is required to detect faults faster and notify the faults to routing protocols immediately. If BFD is associated with OSPF, once a fault occurs on a link between neighbors, BFD can speed up the OSPF convergence.

Table 5-10 Comparison before and after BFD for OSPF is enabled

Associated with BFD or Not

Link Fault Detection Mechanism

Convergence Speed

Not associated with BFD

An OSPF Dead timer expires. By default, the timeout period of the timer is 40s.

At the second level

Associated with BFD

A BFD session goes Down.

At the millisecond level


Figure 5-8 BFD for OSPF

The principle of BFD for OSPF is shown in Figure 5-8.

  1. OSPF neighbor relationships are established between these three routers.

  2. After a neighbor relationship becomes Full, this triggers BFD to establish a BFD session.

  3. The outbound interface on RouterA connected to RouterB is GE 2/0/0. If the link fails, BFD detects the fault and then notifies RouterA of the fault.

  4. RouterA processes the event that a neighbor relationship becomes Down and re-calculates routes. After calculation, the outbound interface is GE1 /0/0 passes through RouterC and then reaches RouterB.

Updated: 2019-12-27

Document ID: EDOC1000174069

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