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CLI-based Configuration Guide - Reliability

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R010

This document provides guidance for configuring reliability services, including interface backup, BFD, VRRP, and EFM.

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Generally, Intermediate System to Intermediate System (IS-IS) sends Hello packets in 10 second intervals. The holdtime of neighbors is three times the interval that Hello packets are sent. If the Router does not receive a Hello packet from its neighbor within the holddown time, the Router deletes the neighbor relationship. This means that the Router detects neighbor faults at the second-level, which leads to significant packet loss on a high-speed network.

In BFD for IS-IS, BFD session establishment is triggered dynamically by IS-IS and is not configured manually. When detecting a fault, the BFD session notifies IS-IS through the RM. IS-IS will then:
  • Process the neighbor Down event.
  • Quickly update the link state PDU (LSP).
  • Perform the partial route calculation (PRC).

This speeds up IS-IS route convergence. BFD does not replace the Hello mechanism of IS-IS, but instead works with IS-IS to quickly detect link faults and to immediately notify IS-IS of route recalculation, guiding packet forwarding. Table 6-2 lists IS-IS convergence speed.

Table 6-2  IS-IS convergence speed

BFD Session

Link Fault Detection Mechanism

Convergence Speed

Not bound

Hello mechanism

At the second level


BFD session in Down state

At the millisecond level


Figure 6-9  BFD for IS-IS

In Figure 6-9, IS-IS is enabled on the devices and an association between BFD and IS-IS is enabled on RouterA and RouterB. When the link between RouterA and RouterB fails, BFD can quickly detect the fault and report the fault to IS-IS. IS-IS then disconnects the neighbors of this interface, which triggers topology calculation. IS-IS updates LSPs so that the neighbors, for example, Router B's neighbor RouterC, can receive the updated LSPs from RouterB. This implements IS-IS fast convergence.

Updated: 2019-08-12

Document ID: EDOC1100034076

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