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Generally, the interval at which the Intermediate System to Intermediate System (IS-IS) protocol sends Hello messages is 10 seconds. If a device does not receive any Hello message from its neighbor within three Hello intervals, the device deletes the neighbor. Therefore, it takes a device a number of seconds to detect that a neighbor is Down. This leads to the loss of a large number of packets in a high-speed network.

In BFD for IS-IS, the establishment of a BFD session is dynamically triggered by IS-IS but not configured manually. When detecting a fault, the BFD session notifies IS-IS of the fault through the Routing Management Module (RM). IS-IS processes the neighbor-Down event and quickly sends the link state PDU (LSP), and performs the partial route calculation (PRC). In this manner, IS-IS routes fast converge.

The BFD fault detection interval is at the millisecond level. Instead of replacing the IS-IS Hello mechanism, BFD works with IS-IS to detect the adjacency fault more quickly. In addition, BFD instructs IS-IS to recalculate routes, ensuring correct packet forwarding.

The RM allows IS-IS and BFD to interact with each other. Through the RM, IS-IS instructs BFD to dynamically set up or delete BFD sessions. The BFD event messages are also delivered to IS-IS through the RM.

BFD for IS-IS Applications

Figure 1 BFD for IS-IS networking diagram 

After BFD is enabled on Switch A, Switch B, and Switch C, the BFD session can quickly detect faults on the link between Switch A and Switch B, and notify IS-IS through the RM. Then, IS-IS sets the neighbor status to Down to trigger the IS-IS topology calculation. In addition, IS-IS updates LSPs to ensure that Switch C (Switch B's neighbor) can receive the updated LSPs from Switch B in time. This implements fast network topology convergence.

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