Specifications

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2.8 Clock Source
When configuring the Juniper Mxxx routers, you can configure the transmit clock
on each interface and you also can configure the system clock reference clock
source. For both router and interfaces, the clock source can be either the routers
internal stratum 3 clock, which resides on the System Control Board (SCB), or an
external clock that is received from on of the routers interfaces.
For the system reference clock source, you can configure three sources: a
primary source, which is the clock source normally used, and secondary and
tertiary sources to provide two level of backup clock sources. For each of the
three clock sources, you specify whether the clock should be the routers internal
stratum 3 clock or the clock received from one of the routers interfaces. You also
can configure how the router switches between clock sources when the source
currently being used becomes unavailable. The default system clock
configuration is that the internal stratum 3 clock is used as the primary source. If
the internal clock fails, the router, by default, does not switch to another clock
source, but remains at the frequency of the internal clock.
Figure below illustrates the different clock sources for the M40: