Specifications

1-20

Cisco MGX 8800/8900 Series Software Configuration Guide

Release 5.1, Part Number OL-6482-01, Rev. A0, January 25, 2005

Chapter 1 Preparing for Configuration

Guidelines for Creating a Network Clock Source Plan

Figure 1-7 shows an example network clock source topology that uses master clock sources on different

switches.

Figure 1-7 Example Network Clock Source Topology with Two Master Clock Sources

In Figure 1-7, Switches 1 and 2 have BITS devices. Switch 1 operates as the master and distributes its

BITS clock source over NNI trunks to Switches 2 through 4. Switch 2 is the standby master and receives

its primary clock signal over the NNI trunk from Switch 1. As long as Switch 1 and its primary BITS

clock source are operating correctly, the entire network is synchronized to the BITS clock source from

Switch 1.

The secondary clock source for the network is the Switch 2 BITS clock source, and all other switches

are configured to use the NNI trunks from Switch 2 as their secondary clock source. If Switch 1 or its

BITS clock source fails, all the switches start using the clock signals from Switch 2 for network

communications. This configuration preserves network sychronization when either a clock source or a

switch fails.

When a clock source fails and recovers, there are a couple of ways that the switch can revert back to the

recovered clock source. If the revertive option is enabled, the switch can automatically revert back to a

recovered primary source from the secondary source. If failures cause the tertiary clock source (the

internal Stratum-3 clock) to take over, the switch will revert to either a recovered primary or recovered

secondary clock source.

Note Regardless of the setting of the revertive option, the switch does revert back to a recovered primary clock

source if the secondary clock source fails. If the secondary clock source is functioning correctly and the

switch configuration does not support an automatic return to a recovered primary clock source, you can

manually switch back to the primary clock source by reconfiguring that clock source.

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Switch 3 Switch 4

Switch 1 –

Active master

clock source

Switch 2 –

Standby master

clock source

BITS

clock

source

BITS

clock

source

P = Primary clock source

S = Secondary clock source