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timers on all interfaces, OSPF cannot establish the adjacency. Here, you see that the<br />

adjacency to the router at 10.0.1.1 through interface fe-0/0/1 has broken because<br />

the timers are different:<br />

aviva@RouterG> show ospf neighbor<br />

Address Interface State ID Pri Dead<br />

10.0.0.2 fe-1/0/1.0 Full 192.168.17.1 128 38<br />

When the timers are the same, the adjacency comes back up:<br />

aviva@RouterG> show ospf neighbor<br />

Address Interface State ID Pri Dead<br />

10.0.1.1 fe-0/0/1.0 Full 192.168.18.1 128 33<br />

10.0.0.2 fe-1/0/1.0 Full 192.168.17.1 128 37<br />

For data traveling at gigabit rates, even decreasing the OSPF hello timer may not<br />

detect failures fast enough. An alternative to adjusting the OSPF timer intervals is to<br />

use the BFD, which detects communication failures with a forwarding-plane next<br />

hop. BFD is useful on interfaces where you can’t detect failure quickly, such as<br />

Ethernet interfaces. Other interface types, such as SONET interfaces, already have<br />

built-in failure detection, so you don’t need to use BFD.<br />

BFD is a simple hello protocol. A pair of systems exchange BFD packets periodically,<br />

and if a system stops receiving the packets for long enough, some component<br />

in that particular bidirectional path to the neighboring system is assumed to have<br />

failed. If you want to shorten the OSPF link failure detection time to 1.5 seconds, set<br />

the BFD packet exchange interval to 500 milliseconds:<br />

[edit protocols ospf area 0.0.0.0]<br />

aviva@RouterG# set interface fe-0/0/1 bfd-liveness-detection minimum-interval 500<br />

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

link detection failure time. Configuring an interval of 500 milliseconds results in a<br />

failure time of 1.5 seconds.<br />

You also need to configure BFD on the interface at the other end of the link. Unlike<br />

the OSPF timers, you do not have to enable BFD on all interfaces in the area. Use the<br />

show bfd session command to see BFD information:<br />

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

Transmit<br />

Address State Interface Detect Time Interval Multiplier<br />

10.0.1.1 Up fe-0/0/1.0 1.500 0.500 3<br />

Client OSPF, TX interval 0.500, RX interval 0.500, multiplier 3<br />

Session up time 00:13:02<br />

Local diagnostic None, remote diagnostic None<br />

Remote heard, hears us<br />

10.0.0.2 Up fe-1/0/1.0 1.500 0.500 3<br />

Client OSPF, TX interval 0.500, RX interval 0.500, multiplier 3<br />

Session up time 00:01:25<br />

Local diagnostic None, remote diagnostic CtlExpire<br />

Remote heard, hears us<br />

2 sessions, 2 clients<br />

Cumulative transmit rate 4.0 pps, cumulative receive rate 4.0 pps<br />

410 | Chapter 12: OSPF<br />

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

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

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