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Solution<br />

Have IS-IS perform the SPF calculation more often:<br />

[edit protocols isis]<br />

aviva@RouterG# set spf-delay 100<br />

<strong>Discussion</strong><br />

One of the most important factors that affects the convergence of IS-IS routes is how<br />

often the software performs the SPF calculation. By default, IS-IS performs the SPF<br />

calculation 200 milliseconds after a topology change is detected. The command in<br />

this recipe halves the time to 100 milliseconds:<br />

[edit protocols isis]<br />

aviva@RouterG# set spf-delay 100<br />

Another way to improve link failure detection times is to use the Bidirectional Forwarding<br />

Protocol (BFD), which provides a mechanism to detect communication failures<br />

with a forwarding-plane next hop. BFD is a simple hello protocol. A pair of<br />

systems exchange BFD packets periodically, and if a system stops receiving the packets<br />

for long enough, some component in that particular bidirectional path to the<br />

neighboring system is assumed to have failed. If you want to shorten the IS-IS link<br />

failure detection time to about 1 second, set the BFD packet exchange interval to 333<br />

milliseconds:<br />

[edit protocols isis]<br />

aviva@RouterG# set interface fe-0/0/1 bfd-liveness-detection minimum-interval 333<br />

By default, BFD multiplies the packet-exchange interval by three to determine the<br />

link detection failure time. Configuring an interval of 333 milliseconds results in a<br />

failure time of just under 1 second.<br />

You also need to configure BFD using the same command on the interface at the<br />

other end of the link. Use the show bfd session command to see BFD information:<br />

aviva@RouterG> show bfd session detail<br />

Address State Interface Detect Time Interval Multiplier<br />

10.0.1.1 Up fe-0/0/1.0 0.999 0.333 3<br />

Client ISIS L2, TX interval 0.300, RX interval 0.300, multiplier 3<br />

Session up time 00:00:37<br />

Local diagnostic None, remote diagnostic None<br />

Remote heard, hears us<br />

1 sessions, 1 clients<br />

Cumulative transmit rate 3.0 pps, cumulative receive rate 3.0 pps<br />

If the link goes down, the BFD session fails and you no longer see the link:<br />

aviva@RouterG> show bfd session detail<br />

Transmit<br />

Address State Interface Detect Time Interval Multiplier<br />

0 sessions, 0 clients<br />

Cumulative transmit rate 0.0 pps, cumulative receive rate 0.0 pps<br />

376 | Chapter 11: IS-IS<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.

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