BFD for OSPF

Definition

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

To be specific, BFD detects the connectivity of a data protocol along 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, which speeds up OSPF's response to network topology changes.

Purpose

A link fault or a topology change causes routers to recalculate routes. Routing protocol convergence must be as quick as possible to improve network availability.

Link faults are inevitable, and therefore a solution must be provided to quickly detect faults and notify routing protocols. BFD for Open Shortest Path First (OSPF) associates BFD sessions with OSPF. After BFD for OSPF is configured, BFD quickly detects link faults and notifies OSPF of the faults. BFD for OSPF accelerates OSPF response to network topology changes.

**Table 1** OSPF convergence speeds before and after BFD for OSPF is configured
Item	Link Fault Detection Mechanism	Convergence Speed
BFD for OSPF is not configured.	An OSPF Dead timer expires.	Second-level
BFD for OSPF is configured.	A BFD session goes Down.	Millisecond-level

Principles

Figure 1 BFD for OSPF

Figure 1 shows a typical network topology with BFD for OSPF configured.

OSPF neighbor relationships are established between these three devices.
After a neighbor relationship becomes Full, a BFD session is established.
The outbound interface on Device A connected to Device B is interface 1. If the link between Device A and Device B fails, BFD detects the fault and then notifies Device A of the fault.
Device A processes the event that a neighbor relationship goes Down and recalculates routes. The new route passes through Device C and reaches Device A, with interface 2 as the outbound interface.