Specifications

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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
Note In releases prior to Release 5, the revertive option applied BITS clock sources and not to clock sources
from trunks. Tertiary clock sources revert to a recovered primary or secondary clock in all releases.
To develop a network clock source plan, create a topology drawing and identify which switches serve
as active and standby master clock sources. For each switch that receives clock sources from other
switches, indicate the lines that carry the primary and secondary clock signals.
Consider the following information when you create your manual network clock source plan:
• Master clock sources that are located near the center of the network minimize clock signal
propagation delay.
• BITS clock interfaces receive Stratum-3 or higher clock signals.
• Configuring a primary and secondary clock source provides fault tolerance.
• If both the primary and the secondary external clock sources fail, the switch uses an internal
Stratum-3 clock.
• When using an external clock source and redundant PXM cards, use a Y-cable to connect that clock
source to the same clock port on both PXM cards. Do not run separate external clock sources to each
card as this can produce timing problems.
• If the switch is using its own internal Stratum-3 clock and a primary or secondary clock source
recovers, the switch will use the recovered clock source.
• If no primary or secondary clock sources are configured, the switch uses the internal Stratum-3
clock.
• Primary and secondary BITS clocks can be configured after the switch is initialized. For more
information, see the “Configuring Clock Sources” section in Chapter 2, “Configuring General
Switch Features.”
• Primary and secondary NNI trunk clocks must be configured after the cards and lines are configured.
For more information on configuring a switch to use a clock source transmitted over a PXM1E line,
see Chapter 3, “Provisioning PXM1E Communication Links.” For more information on configuring
a switch to use a clock source transmitted over a service module trunk, refer to the appropriate
service module book. The service module books are listed in Table 1-1.
Planning for NCDP Synchronization
The MGX switches support a Network Clock Distribution Protocol (NCDP), which selects the best clock
in your network based on your configuration, and automatically configures the path to that clock for each
node throughout your network. In an NCDP clock configuration, there are no primary and secondary
clock sources. Instead, you configure several clock sources for the nodes in your network, from which
NCDP selects the best (or root) and second best clock source for the network.
Once NCDP has selected the root clock source, it is propagated to all the nodes in the network so that
all nodal clocks are synchronized. If the root clock source fails, the second best clock source becomes
the root clock source. If the second best clock source fails, NCDP selects the third best clock source to
take over as the root clock source, and so forth.
If you want to use NCDP to set up your network clocks, you must first enable NCDP, as described in the
“Managing NCDP Clock Sources” section in Chapter 9, “Switch Operating Procedures.” Once you
enable NCDP on your node, it is automatically enabled on all NNI ports on the node. When NCDP is