BFD for RIP

A link fault or topology change causes routers to recalculate routes. Therefore, route convergence must be quick enough to ensure network performance. A solution to speed up route convergence is to quickly detect faults and notify routing protocols of the faults.

Bidirectional Forwarding Detection (BFD) detects faults on links between neighboring routers. Associated with a routing protocol, BFD can rapidly detect link faults and report the faults to the protocol so that the protocol quickly triggers route convergence. Traffic loss caused by topology changes is minimized. After RIP is associated with BFD, BFD rapidly detects link faults and reports the faults to RIP so that RIP quickly responds to network topology changes.

Table 3-1 lists the link fault detection mechanisms and convergence speed before and after BFD is associated with RIP.

Principle

BFD is classified into static BFD and dynamic BFD:

• Static BFD

  In static BFD, BFD session parameters (including local and remote discriminators) are set manually using commands, and BFD session setup requests are manually delivered.

• Dynamic BFD

  In dynamic BFD, BFD session setup is triggered by routing protocols. The local discriminator is dynamically allocated and remote discriminator is obtained from the peer. A routing protocol notifies BFD of the neighbor parameters (including destination and source addresses), and then BFD sets up a session based on the received parameters. When a link fault occurs, the protocol associated with BFD quickly detects that the BFD session is Down, and switches traffic to the backup link. This feature minimizes data loss.

A device can implement static BFD even if the peer device does not support BFD. Dynamic BFD is more flexible than static BFD.

Application

After RIP is associated with BFD, BFD reports link faults to RIP within several milliseconds. The RIP router then deletes the faulty links from the local routing table and starts the backup link. This feature increases route convergence speed.

Figure 3-8 RIP and BFD association network

Implementation of RIP and BFD association:

• As shown in Figure 3-8, RouterA, RouterB, RouterC, and RouterD set up RIP neighbor relationships. RouterB is the next hop on the route from RouterA to RouterD. RIP and BFD association is configured on RouterA and RouterB.
• When the link between RouterA and RouterB is faulty, BFD quickly detects the fault and notifies RouterA of the fault. RouterA deletes the route with RouterB as the next hop, and then recalculates a route. The new route passes RouterC and RouterB and reaches RouterD.
• When the link between RouterA and RouterB recovers, a session is set up again. RouterA receives routing information from RouterB and selects the optimal route.