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Configuration Guide for Cisco DSLAMs with NI-2

OL-2074-03

Chapter 3 Initially Configuring the Cisco DSLAM

Configuring Network Clocking

Figure 3-3 Transmit Clock Distribution

Because the port providing the network clock source could fail, Cisco IOS software provides the ability

to configure additional interfaces as clock sources with priorities 1 to 4.

If the network clock source interface stops responding, the software switches to the next

highest-configured priority network clock source. For example, Figure 3-4 shows:

• DSLAM number two is configured to use ATM 0/2 as its highest-priority clock source, and an

external reference clock as its second highest-priority clock source.

• ATM 0/1 uses network-derived transmit clocking.

• ATM 0/2 fails.

• The external reference clock becomes the active clock signal and is distributed to the WAN ports

and line cards. ATM 0/1 uses the external reference clock.

• If the configuration option network-clock-select revertive is set, the DSLAM continuously attempts

to revert to the valid clock source with the highest priority. To be considered valid, a clock source

must remain stable for a least 1 minute.

Internal reference

source

Primary NI-2 card

Loop reference clock

Bits input

Trunk Rx

Trunk TX

IOS

processor

and switch

fabric

Primary

NTR

reference

Clock

detection

Line card