Cisco MGX 8830, Configuration Manual

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Cisco MGX 8850 (PXM1E/PXM45), Cisco MGX 8950, Cisco MGX 8830, and Cisco MGX 8880 Configuration Guide
Release 5.0.10, OL-3845-01 Rev. B0, August 16, 2004
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.
In Figure 1-7 , 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
P
P
S
S P
S P
S