Cisco Gigabit Switch Router 12008 Installation And Configuration Manual page 70

Overview of the Cisco 12008
In a redundant CSC configuration, the phase of the master clock generator on one card
is synchronized with that of the other card. If either clock drifts, the master clock
generators on both cards remain tightly aligned.
Should one of the CSCs fail, the phase lock between the two master clock sources is
aborted within nanoseconds, enabling the surviving CSC clock to remain stable and take
over master clock duties.
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Frame synchronization generator—This function on the CSC provides a periodic signal
to the line cards and switch cards to control data flow.
In a redundant CSC configuration, either CSC can adopt the frame synchronization
phase of the other to ensure phase alignment. The line cards can switch between frame
synchronization masters without disruption.
If the frame synchronization function on one CSC fails, cutover to the surviving frame
synchronization generator on the other card occurs within nanoseconds, sustaining
system operations.
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Central switch allocator and scheduler—This function on the CSC allocates switching
resources to line cards and schedules (arbitrates) the flow of data through the router's
switch fabric.
Switch arbitration begins with a set of requests from line cards to send data through the
router's switch fabric. The scheduler plans a set of paths through the switch fabric to
carry as much data as possible per unit of time. At the next available time unit, the
request to send data is granted, and the data is sent to its destination. The next round of
switch arbitration (scheduling) then begins.
The scheduler also sends switch fabric control information to each switch plane to
create appropriate data paths through the switch fabric. When the new data paths are
configured into the switch fabric, data begins to flow toward the destination line card(s).
The central switch allocator and scheduler accepts data transport requests from all line
cards (including the RP), generates grants (accepted data transport requests), and drives
all planes of the router's switch fabric.
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Single plane switch fabric—The CSC's single-plane switch fabric provides an OC-12
rate of switching capacity for the router. This switch fabric plane operates under control
of the CSC's central switch allocator and scheduler.
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Cisco 12008 Gigabit Switch Router Installation and Configuration Guide