Fore Systems forerunner series, Configuration Manual

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ForeRunner ATM Switch Network Configuration Manual
Configuring Timing
9.3 Switchclock
If a TCM is not installed in the switch, all of the ports within a fabric use the switchclock as
their timing reference. The switchclock can be any port that is able to recover a clock (i.e., the
network module supports distributed timing). (To see if the network module supports distrib-
uted timing, use the command
conf module show
and look for
yes
under the
Timing
field.)
In an ASX-1000, the port may be from another fabric in the same chassis.
When the switchclock is set to any of the ports within the same fabric, that port’s clock is
exported to all of the network modules within the same fabric. On an ASX-1000 only, the
switchclock is also exported to the network modules in all of the other fabrics in this chassis.
You can then configure each of the other fabrics to use that clock. In this way, all of the net-
work modules in all of the fabrics will use the same timing source.
9.3.1 Failover of the Switchclock
You can configure a primary switchclock and a secondary switchclock. If the primary clock is
valid, it is used. If the primary switchclock fails, the secondary clock is used. When the pri-
mary clock returns, it is used again as the clock source.
If both the primary and secondary values fail, the switch fabric uses the crystal of the first
available timing network module as the switchclock, going from A to D. For example, if net-
work module A supports distributed timing, then the crystal from A is used as the switch-
clock. As another example, if network module A is not installed, and network modules B and
C do not support distributed timing, but network module D does support distributed timing,
then the crystal from D is used as the switchclock.
Failover of the switchclock happens automatically when any of the following events occur:
• A carrier detect or carrier loss occurs on any port in this switch fabric.
• A network module is swapped in or out.
• A switch fabric is swapped in or out.