Interface and Router Clock Sources

When running a loopback test on T1, T3, ATM, and SONET interfaces, you must configure the transmit clock. The transmit clock aligns each outgoing packet transmitted over the router's interfaces. For both the router and interfaces, the clock source can be the router's internal Stratum 3 clock, which resides on the System Control Board (SCB), the System and Switch Board (SSB), the Forwarding Engine Board (FEB), or the Miscellaneous Control Subsystem (MCS) (depending on the router model), or an external clock that is received from the interface you are configuring. For example, interface A can transmit on interface A's received clock (external, loop timing) or the Stratum 3 clock (internal, line timing). Interface A cannot use a clock from any other source.

By default, each interface uses the router's internal Stratum 3 clock. To configure the clock source of each interface, include the clocking statement at the [edit interfaces interface-name] hierarchy level:

[edit interfaces interface-name]

clocking (internal | external);

Figure 2 illustrates the different clock sources.

Figure 2: Clock Sources