Specifications

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Cisco MGX 8850 (PXM45) and MGX 8950 Software Configuration Guide
Release 3, Part Number 78-14788-01 Rev. C0, January 2004
Chapter 1 Preparing for Configuration
Collecting Information
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.
• Multiple master clock sources provide fault tolerance.
• If both primary and 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 1, “Preparing for
Configuration.”
• Primary and secondary AXSM clocks must be configured after the AXSM cards and lines are
configured. For more information, see “Configuring AXSM Line Clock Sources” in Chapter 6,
“Provisioning AXSM Communication Links.”
Planning for NCDP Synchronization
Release 3 of the MGX switches supports a network clock distribution protocol (NCDP), which selects
the best clock in your network for synchronization, 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 the NCDP protocol, as
described in “Managing NCDP Clock Sources” in Chapter 7, “Switch Operating Procedures.” Once you
enable NCDP on your node, it is automatically enabled on all NNI ports on the node. When NCDP is
enabled, a root clock source is automatically selected and distributed to all nodes in the network that
have NCDP enabled. NCDP automatically selects an internal oscillator on one of the NCDP nodes to be
the root clock source. Each NCDP node in the network is synchronized to this root clock reference. If
you do not want the root clock source to be an internal oscillator, you can configure it to come from an
external source with the cnfncdpclksrc command, as described in Chapter 7, “Switch Operating
Procedures”, in the section “Configuring an NCDP Clock Source”
NCDP uses the following criteria to finding the best root clock source for the network:
• Priority (should be sufficient to find the root)
• Stratum level (should be sufficient as a tie-breaker)
• Clock source reference
• ATM address of the switch